You just Close out any line item not needed and/or the entire PO. They don't Have to be linked; it is just Nice, when you need to research fulfillment.
"When I received the rest of the order I apparently forgot to tab thru
the remaining parts that arrived and they weren't assigned to that PO."
Instead of starting from the PO, if you use that vendor name on a Bill, an Item Receipt (which is a bill as a preliminary status), a Check, or a Credit Card, you get a Pop Up reminding you about the PO, but this might be a different Purchase, of course.
