Incomplete Purchase Order

Hi gurus, Can anyone explain what is incomplete purchase order.

I created a shopping cart with some catalog items.. For some catalog items the supplier information is not available..

That shopping cart i didn't find in SOCO, but incomplete po is created and i checked the status using BBP_PD.

What does the status HELD, CREATED, INCOMPLETE means.. What is the next process i need to do for incomplete PO. Is that necessary to change incomplete PO to a completed PO???

Thanks in advance