A purchase order number is the unique reference a supplier quotes back on the delivery note and invoice, so the bill can be matched to the order. These are small details and they are the ones that decide whether the rest of the process works, because a purchase order that cannot be referenced, whose terms were agreed verbally, or that covers three suppliers at once is a document that cannot be matched to anything.
The number is the reference everything else hangs on
A purchase order number is what a supplier quotes on the delivery note and the invoice so the bill can be tied back to something you agreed. On the worked example order 1042 was agreed at $827.21, which means an invoice quoting 1042 can be checked in seconds. Without the number the same check depends on what somebody remembers.
Sequential, unique, and never reused
There is no advantage to encoding the year, the department or the supplier. Sequential is enough and simple beats clever. The two rules that matter are that a number is never reused, even for a corrected order, and that it appears where the supplier will find it and quote it back.
Payment terms belong on the order, not in an email
Net 30, net 15, a deposit, or payment on delivery: whichever it is, stating it on the order is what makes an early demand for payment a departure from something agreed rather than a difference of recollection. It is one line and it is left off most hand-written orders.
One order, one supplier
A vendor purchase order is simply a purchase order issued to a named supplier, and the useful discipline is that an order names exactly one. Splitting a mixed order is a minute of work and it is what keeps matching simple, because every invoice then relates to exactly one order.
Questions people ask about what is a purchase order number
What is a purchase order number?
The unique reference on an order that a supplier quotes on the delivery note and invoice, so a bill can be matched to the commitment it relates to.
What payment terms should a purchase order carry?
Whatever you agreed, stated explicitly: net 30, net 15, a deposit, or payment on delivery. The point is that it is on the document rather than in a conversation.
Should one order cover several suppliers?
No. One order, one supplier. It takes a minute to split and it keeps matching simple, because every invoice then relates to exactly one order.