A purchase order and an invoice describe one transaction from two sides. The order is written by the buyer before anything happens and records what was agreed. The invoice is written by the supplier afterwards and claims payment for what they say was supplied. The gap between the two is what a payables team examines every day.
The PO: yours, and in advance
Items, quantities, unit prices, delivery address and date, your terms, and a unique number. It records the agreement and creates a commitment once the supplier acts on it, which is why approval belongs before it is issued. It is also your only evidence of what was agreed, so the care taken over it determines how defensible your position is later.
The invoice: theirs, and afterwards
What was supplied, what is owed, by when, with their reference and ideally your order number. It is a claim rather than a fact, which is the shift that distinguishes business payables from consumer payment. Claims get verified before they are settled, and the verification is what the whole function exists to perform.
Where they legitimately differ
Freight added at billing, tax, part deliveries invoiced separately, and prices renegotiated after the order was raised. All normal. Tolerance absorbs the small differences and dated revisions record the agreed ones, which keeps the exception queue for genuine disagreements rather than for everyday commercial reality.
And the document that settles it
The goods receipt. An order and an invoice agreeing proves the supplier billed what you asked for and proves nothing about delivery. Only a record of what arrived closes that gap, which is why the standard control for physical goods compares three documents and not two.
Questions people ask about invoice vs po
Which one governs the payment terms?
Both may state them. Terms on your order are what you rely on when the supplier's invoice states something different, which is common because their template prints their standard terms automatically.
Can one PO cover several invoices?
Yes, for staged deliveries or ongoing services, which is exactly why line-level matching with running quantities matters.
What if there is no PO?
The invoice must be approved on its own merits by whoever authorised the spend, which is the most expensive path an invoice can take.