Key takeaways
- A purchase order formalises an agreement before delivery: who orders what, in what quantity and at what agreed price.
- It is not mandatory and is not a tax document, but it binds both parties as soon as it is accepted.
- The invoice, by contrast, is the tax document: it carries the VAT details and the e-invoicing mandate.
- The order number flows into the invoice, in the purchase order reference field (BT-13) of the EN 16931 standard.
The purchase order, the written trace of an agreement
Before an invoice goes out, a transaction almost always starts with an agreement: the buyer states what they want, the supplier confirms the price and the lead time. The purchase order is the written trace of that agreement. It sets down in black and white what has been ordered, in what quantity and at what price, before any goods move or any service starts.
This document is in no way a tax formality. Nothing obliges a Belgian business to issue a purchase order, and it never replaces the invoice. Its strength lies elsewhere: as soon as both parties accept it, it stands as agreement on the order and serves as a common reference point through to payment. When handled well, it is also what lets you check that you received what you ordered, and were then invoiced for what you received.
This article sets out what the purchase order really does, what it contains, how it differs from the invoice, and how its number flows into the e-invoice.
Purchase order and invoice: two documents, two functions
The most common confusion is to treat the purchase order and the invoice as interchangeable. They are not. The purchase order looks forward: it describes an order that has not yet been carried out. The invoice looks back: it records a completed delivery or service and claims payment for it.
The difference that matters most is fiscal. The invoice is a regulated document: it must carry the mandatory VAT details and, in a B2B relationship, comply with the e-invoicing mandate. The purchase order is subject to none of those rules. It can stay a simple commercial document, though reducing it to that means giving up a valuable control tool.
| Purchase order | Invoice | |
|---|---|---|
| Issued before delivery | ||
| Tax document (mandatory VAT details) | ||
| Subject to the B2B e-invoicing mandate | ||
| Binds the parties once accepted | ||
| Basis for payment |
For the detail of what the invoice must contain, see the mandatory details of an invoice in Belgium. The purchase order follows a looser logic, described below.
What a useful purchase order contains
There is no legal list of details for the purchase order comparable to the one for invoices. But a purchase order that serves no purpose is an incomplete one. To play its role as a common reference, it must make it unambiguous who ordered what, and on what terms.
The information in a usable purchase order
The identity of both parties
Name or company name, address and contact details of the seller and the buyer.
A date and an order number
The number is the key that will later link the delivery and the invoice back to this order.
The detail of the items or services
Description, quantity, agreed unit price and, where relevant, discounts.
The agreed terms
Lead time, payment terms and conditions, place of delivery.
The order number deserves particular attention. It is what turns an isolated document into a link in a chain: it will reappear on the delivery note, then on the invoice, and it is through it that the accounting team matches the three. An order with no clear number is an order that is hard to control.
The order reference that links the purchase order to the invoice
The link between the purchase order and the invoice is not just an internal good practice: it is provided for by the very format of the e-invoice. The European standard EN 16931, the foundation of e-invoicing, reserves a dedicated field for the buyer's order number: the purchase order reference, identified as BT-13. In an invoice using the Peppol BIS Billing 3.0 profile, that field maps to the OrderReference element.
In practice, the supplier carries the number of the received purchase order into their invoice. On the buyer's side, the software reads that field and automatically matches the invoice to the original order, with no re-keying. This is what makes possible the "order - receipt - invoice" check that procurement teams run, and it cuts disputes over what was actually ordered.
order reference
the EN 16931 field that carries the order number
linked documents
order, delivery then invoice, through one number
re-keying
matching happens on the reference
The electronic order on the Peppol network
The invoice is not the only document Peppol can carry. The network also defines electronic order profiles: a simple order profile (Order only) and an order-with-response profile (Ordering), published by OpenPeppol as part of post-award coordination. The order can therefore travel in structured form, exactly like the invoice, and trigger a response from the supplier.
In Belgium, though, keep a sense of proportion. The e-invoicing mandate between taxable businesses, which takes effect on 1 January 2026, applies to the invoice, not to the order. The electronic order stays optional. It does, however, offer a real benefit for high-volume flows: order, confirmation and invoice then travel within the same structured framework, and matching on the order reference becomes fully automatic.
Link your orders to your invoices, with no re-keying
YouInv carries the order reference into your compliant e-invoices and sends them over Peppol in a few clicks.
What this changes for your operations
For a small business, the purchase order is in no way mandatory, and many operate without it. But as soon as volumes rise or customers are themselves structured businesses, it becomes a safety net: it is the document that proves what was agreed, and the key that links the invoice to that agreement.
The habit to build is simple: give every order a clear number, and carry it into the matching invoice. Invoicing software that handles the order reference and places it in the right field of the e-invoice saves you the re-keying and speeds up payment, because the customer no longer has to work out which order your invoice relates to.
Further reading
- The mandatory details of an invoice in Belgium: what the invoice, in turn, must contain.
- From quote to invoice: managing your quotes: the step before the order.
- Electronic ordering and catalogues on Peppol: the network's other documents.
The reference source prevails: the documentation of the order reference field (BT-13) and the Peppol order profile published by OpenPeppol.
Is a purchase order mandatory in Belgium?
No. Nothing forces a business to issue a purchase order: unlike the invoice, it is not a tax document. It is a commercial and contractual tool. Once both parties accept it, it stands as agreement on the order and serves as a reference throughout the transaction.
What is the difference between a purchase order and an invoice?
The purchase order formalises an agreement before delivery: it states what is ordered, in what quantity and at what agreed price. The invoice is issued after the service or delivery; it is the tax document, subject to the mandatory VAT details and, for B2B, to the e-invoicing mandate.
What details should a purchase order include?
There is no legal list like the one for invoices, but a useful purchase order identifies the seller and the buyer, carries a date and an order number, describes the items, quantities and agreed prices, and recalls the terms of sale. These details are then used to check the delivery and the invoice.
How do you link a purchase order to its invoice?
By carrying the order number into the invoice. In an e-invoice compliant with the EN 16931 standard, that number sits in the purchase order reference field (BT-13). The buyer can then automatically match the invoice to the original order.
Does the purchase order travel over Peppol?
Yes, Peppol defines electronic order profiles (Order only, Ordering) alongside the invoice. In Belgium, only the e-invoice is mandatory for B2B from 1 January 2026; the electronic order stays optional but lets you automate the whole order-to-invoice chain.
