Guide on How Do ERP Systems Work

What Actually Happens Inside an ERP

ERP is usually described as "an integrated system that connects departments". True, but that description does not explain how the connection works or why it differs from a set of separate programs, one per department. Understanding the mechanics matters to any manager considering an ERP, because it shows what will change in the team's daily work and what has to be prepared before go-live.

This guide explains how an ERP system works from the inside: the shared database, the difference between master data and transactions, document flow, automatic accounting, how modules interact, analytical dimensions, controls and reporting. We trace one sales order and one purchase through the system with real numbers, then cover cloud versus on-premise and the points specific to Saudi Arabia.

Principle One: A Single Database

Picture a company running an accounting package, a separate inventory program, sales spreadsheets and a payroll file. Each has its own customer, item or employee list. A customer's address changes in one place and not the others. A sale is logged in sales, re-keyed into inventory, then re-keyed into accounting. Every hand-off is a chance for error or delay, and at month-end the sales report and the ledger show different revenue.

An ERP removes the problem at its source: every module reads from and writes to one database. A customer exists once, an item exists once, an employee exists once. When sales issues an invoice, it does not "send" it to accounting; the invoice itself becomes part of the data accounting reads. There is only one version of the truth.

Master Data and Transactions

The database holds two kinds of data, and the distinction explains many implementation problems:

When a salesperson raises an invoice, they do not type the customer's name, VAT number and payment terms; they pick the customer and those details come with it. Picking an item brings its unit of measure, tax category and linked GL account. So a single master data error, a wrong VAT number or a wrong account on an item group, repeats in every transaction that follows. That is why master data cleansing before go-live is one of the most important phases of any implementation.

Document Flow

Documents are not created in isolation; each one is built from the one before. In the sales cycle:

1. Quotation: items, quantities and prices for the customer.
2. Sales order: created from the accepted quote without re-entry, reserving stock.
3. Delivery note: created from the order at dispatch, taking stock out of the warehouse.
4. Invoice: created from the delivery note for quantities actually delivered, not quantities ordered.
5. Receipt: matched to the invoice when the customer pays, closing its balance.

Two things follow. Data is entered once, at the start of the chain. And there is a complete audit trail: from any invoice you can drill back to the delivery, order and quote, and see who created each and when. Purchasing works the same way: requisition, purchase order, goods receipt, supplier invoice, payment.

Automatic Accounting

This is the biggest difference from departmental software. Every document with a financial effect generates its journal entry automatically, based on posting rules set up once during implementation. Accountants do not write sales, cost of sales or goods receipt entries; they review them and handle exceptions.

Posting rules define which revenue account applies to each item group, which inventory account to each warehouse, which tax account to each tax category, which cost center to each branch. The quality of this setup at the start of the project decides the quality of financial reporting for years.

Worked Example 1: One Sales Order

A Riyadh trading company sells 10 units of a device to a business customer at SAR 1,000 each before VAT. Weighted average cost is SAR 700 a unit. Figures are hypothetical.

Order confirmed: the system checks that 10 units are available, reserves them so they are not sold twice, and checks the customer's credit limit. No journal yet, because nothing financial has happened.

Goods delivered: warehouse stock drops by 10 and the cost entry posts automatically:

Invoice issued: VAT at 15% is SAR 1,500, and revenue posts:

Because the buyer is a business, the e-invoice goes to the Fatoorah platform for clearance before it is shared with the customer, as Phase 2 requires.

Payment received: a receipt of SAR 11,500 is matched to the invoice; the entry moves the amount from receivables to bank and closes the invoice.

Four steps by sales, warehouse and collections staff updated stock, the available-to-sell report, item profitability (SAR 3,000 gross margin), customer ageing, the VAT report and the financial statements, without an accountant writing a single entry.

Worked Example 2: A Purchase With a Price Difference

The same company orders 50 units from a supplier at SAR 700 before VAT. On arrival the storekeeper records a goods receipt for 50; stock rises and the system posts debit inventory SAR 35,000, credit a clearing account usually called "goods received not invoiced". That clearing account means stock appears in the books the moment it arrives, before the invoice.

The supplier's invoice arrives at SAR 720 a unit, SAR 36,000 before VAT. The three-way match fails because the invoice price does not match the PO, so the system holds the invoice until someone decides: was the increase agreed, in which case the PO is amended, or is it a supplier error needing a corrected invoice? With separate systems, that invoice could easily have been booked and paid with nobody noticing the extra SAR 1,000.

Once corrected to SAR 35,000, the invoice posts: debit clearing SAR 35,000, debit input VAT SAR 5,250, credit supplier SAR 40,250. Payment then moves the balance from the supplier to bank. Every step links to the one before, so from the supplier balance you can drill to the invoice, receipt and PO.

Corrections Without Deletion

Well-designed accounting systems do not delete posted documents. A wrong invoice is reversed with a credit note, fully or partly, and reissued if needed. A wrong receipt quantity is fixed with a correction or return document. Slower than deleting, but the audit trail survives: who made the mistake, when, and how it was fixed. E-invoicing expects this too, since credit and debit notes follow the same path as the invoice they relate to.

Period locking adds another layer. Once a month is closed, nothing can be posted into it without special rights, so figures already reported to management or used in a tax return cannot quietly change.

Month-End in an Integrated System

Since entries post with transactions, month-end shifts from assembling numbers to reviewing them. A typical checklist becomes short:

1. Review the goods-received-not-invoiced account and chase missing supplier invoices.
2. Run and review automatic fixed asset depreciation.
3. Post accruals and prepayments that do not come from an operational document.
4. Reconcile bank accounts, faster because receipts and payments carry references.
5. Check the VAT report against VAT accounts in the general ledger.
6. Lock the period and produce the statements.

Analytical Dimensions: How Branch Profit Appears

The chart of accounts answers "what kind of income or expense?" but not "where?" or "for which project?". So each transaction carries extra dimensions such as branch, cost center, project and department, often inherited automatically: an invoice raised by a Jeddah user carries the Jeddah branch, materials issued to a project carry the project code, warehouse payroll lands on the warehouse cost center.

That means a P&L per branch or project needs no manual allocation spreadsheets at month-end; it is built from the transactions themselves. It also keeps the chart of accounts lean: one rent account plus a branch dimension, rather than "Rent Jeddah" and "Rent Dammam" as separate accounts.

How the Modules Interact

Purchasing, inventory and accounting

A receipt against a PO raises stock and updates item cost; the supplier invoice is matched to PO and receipt before the payable is created. Any mismatch stops the invoice for review.

Inventory and purchasing

When an item falls below the reorder point set on its record, the system alerts or proposes a requisition, cutting stockouts without constant manual checking.

HR, payroll, bank and accounting

Attendance and leave feed payroll, which calculates pay, allowances, deductions and social insurance contributions, produces the bank transfer file, and posts payroll cost to the right accounts and cost centers.

Manufacturing, inventory and purchasing

A production order reads the bill of materials, reserves or issues raw materials, proposes purchases for shortages, and on completion receives finished goods at a cost built from materials, labour and overhead.

Projects and everything else

For contractors and service firms, purchases, materials, hours and invoices are linked to a project, so cost, revenue and margin are visible at any time, not only after closure.

For what each module does in detail, see our article on ERP software modules.

Roles, Permissions and Approvals

One database does not mean everyone sees everything. The system runs on roles: a storekeeper records receipts and issues without seeing prices, a salesperson creates orders for their own branch's customers, an accountant posts manual journals but cannot edit approved invoices. Approval workflows ensure a PO above a threshold passes the department head and then the finance manager before release. Internal control is built into how work is done, not left to people remembering the policy.

Why Reporting Becomes Real-Time

With separate programs, a report means exporting from several sources and stitching it together in a spreadsheet, which takes days and is stale on arrival. In an ERP, reports read directly from posted transactions, so a manager sees today's sales, current stock and overdue receivables as they stand now, and can click a figure in a profitability report to reach the invoices behind it.

Cloud vs On-Premise

The mechanics above are identical either way. What differs is where the system runs and who looks after it:

We compare the two in detail in cloud vs on-premise ERP in Saudi Arabia.

Saudi-Specific Points

• E-invoicing: Phase 1 started on 4 December 2021 and Phase 2 has rolled out in waves since 1 January 2023. The ERP must generate invoices in the required format, integrate with Fatoorah, and route credit and debit notes the same way. More in our e-invoicing guide.
• VAT: 15%, with return data drawn directly from transactions.
• Personal data: the PDPL has been in force since September 2023, with enforcement from September 2024 after a one-year grace period. Employee and customer data in the ERP falls under it, which makes permissions and audit logs more important.
• Cloud security: the National Cybersecurity Authority has issued Cloud Cybersecurity Controls; requirements depend on the organisation and data classification, so ask vendors where data is hosted and how they comply.
• HR: links to GOSI, wage protection through Mudad, and Muqeem for iqamas are core HR requirements in the Saudi market.

What This Means for Implementation

• Master data first: give real time to cleaning customer, supplier and item lists before migration.
• Design posting rules carefully: the chart of accounts and account mapping drive report quality for years.
• Record events when they happen: the system assumes receipts are logged on arrival; late entry corrupts stock and everything built on it.
• Expect procedures to change: some manual work disappears and some responsibilities move between departments. Preparing people for that is part of the project.

Tranquil is a cloud ERP covering financial accounting, ZATCA e-invoicing, inventory, sales and distribution, purchasing, HR, payroll, projects, contracting, asset management, property management and POS in one system. Browse the modules on our solutions page.

Frequently Asked Questions

How is an ERP different from accounting software?

Accounting software records the financial effect after the fact, often by manual entry. An ERP records the operations themselves, from quote to receipt, issue and payroll, and generates the accounting entries from them automatically.

Do we need to implement every module at once?

No. Many companies start with accounting, sales, purchasing and inventory and add HR or projects later. What matters is that modules come from the same system and share one database.

Why do report figures change after go-live?

Often because the system exposes what separate files were hiding, such as stock differences or uncollected invoices. Sometimes because opening balances were migrated wrongly, which is why balances are reconciled before launch.

Does a cloud ERP work offline?

It needs an internet connection, because it runs on the vendor's servers. Check connectivity at branches and warehouses before go-live.

How do I know if Tranquil fits my company?

The best test is a demo built on scenarios from your own operations. Contact the Tranquil team to arrange one.