Purchase orders and invoices are two descriptions of the same transaction written by different parties at different times. Yours states what you agreed to buy; theirs claims payment for what they say they supplied. The reference number joins them, and for physical goods a third document is what makes the join worth trusting.
Why the reference matters so much
It is what allows an automatic comparison rather than an investigation. An invoice quoting an order number can be matched in milliseconds; one quoting nothing needs somebody to establish what it is for, who authorised it and which budget carries it. That is why organisations chase suppliers for references and, often, return invoices without them.
Why the receipt matters more
An order and an invoice agreeing proves the supplier billed what you asked for. It proves nothing about whether anything arrived. Only the goods receipt does that, which is why the standard control for physical goods compares three documents. An organisation matching two is trusting the supplier on precisely the point most worth checking.
Where they differ legitimately
Freight added at billing, tax, part deliveries invoiced separately, and prices renegotiated after the order. All normal, none an error. Tolerance absorbs the small differences and dated revisions record the agreed ones, which keeps the exception queue for genuine disagreements rather than for everyday reality.
Questions people ask about purchase orders and invoices
Can an invoice cover several orders?
Yes, and it complicates matching. Encouraging suppliers to invoice per order makes both sides' processing simpler.
Should we refuse invoices without a reference?
Many organisations do, returning them for reissue. Communicate it in advance or it just generates angry calls.
What if the order was never raised?
The invoice is approved on its own merits by whoever authorised the spend, and the count of such invoices is worth reporting by department.