A purchase order is not merely an internal document. Once a supplier receives it and acts on it, a commitment exists in most commercial relationships, whatever your internal process says. That is the whole reason approval belongs before the order is sent, and it is why organisations that approve at the invoice stage have a review process rather than an approval one.
What the order commits
The items, quantities and prices stated, delivered where and when specified, on the terms attached. That is why what the price includes, the units of measure and the delivery date all matter: they are the terms of a commitment, not just fields on a form. Vagueness in them is vagueness in what you have agreed to.
Why terms belong on it
Your payment terms on the order are what you rely on when a supplier's invoice states different ones, which happens often enough to matter. The same applies to delivery expectations. Including them costs a line of text and prevents a category of disagreement that otherwise resolves in favour of whoever argues longest.
Amendments are amendments
Changing an order after issue changes the commitment, so it should be a dated revision rather than an overwrite, and above a threshold it should go back for approval. Orders that grow past their approved value one amendment at a time are a well-known way for authority limits to be bypassed without anybody intending it.
Questions people ask about purchase order agreement
Is a purchase order a contract?
It can form part of one, and how it interacts with a supplier's own terms depends on the circumstances and on your jurisdiction. That is a question for your own legal advice rather than for this page.
What if the supplier sends their own terms back?
That is a commercial and legal question worth resolving before delivery rather than at invoice time. Raise it with whoever advises you.
Should every purchase have an order?
Most organisations set a value threshold. Below it, the control is the invoice approval instead, which is later and weaker.