Hi - Not a dumb question. Order activity rules (OAR) gives flexibility to the process, by Order Type and Line Type. I think of it as "where you are" and "Where you're going" combination. Programs move line items (therefore the status codes) onward. So, after entry (220) what's next?
In your example, "where you are" (280) that's PO Print. Because that's in the "last status" position, I know that step has been completed and you are headed to receipt. "Where you're going" seems to be a receipt variation - 440 for 2-way voucher match (400 is the delivered receipt status for 3-way voucher match).
Because your last status is 280, I know there has not yet been a partial receipt (programs move line items forward). If your line item was (as an example) 440/440, that might be an example of Initial receipt / Partial Receipt awaiting for the remaining balance.
Status codes also allow for filtering for only line items at that status: there might be a non-stock Receipt Match process that has a PO inquiry version that is only filtering for non-stock line items at 440.
I think the complexity comes into play when you have approvals (with rejections), partial receipts, and line items in receipt routing where the process splits then line items. That's where you want variations in your order activity rules for alternate process steps, like 400/400. You might also have variations for line items that go into approval (220/230) and could be rejected (230/220) as well as approved (230/280).
Your stock line item is going to take a different journey than a non-stock, contract line item.
I think PO to AP hand-off can be a bit confusing. Since you're new to it, you can review the process in the PO Ledger. You should see that the Order line item was entered (220/230), approved (230/280), Printed (280/440) and is now waiting to be "matched" in AP. When the vendor sends the invoice, your colleague in AP will create a voucher in JDE and match it to that line item which closes the PO line (440/999). The step that is to be completed next is Voucher Match.
Your purchase order line item will be closed after the voucher is created (as long as it's matched to the order line). The AP voucher advances through AP so that a check is cut to the vendor.
Hope that helps -