F0411 - What values get populated in PKCO, PO, PDCT

What sort of values get stored in the following columns;

PKCO, PO, PDCT

I know these columns store PO info, but I can see in our demo environment only column PO is populated for a specific row and when I want to query that PO it does not return any value from F4311. Thats made me wonder if these columns get populated by any other module as well.

I am trying to find all the AP Vouchers which are not against Purchase Order.

Thanks

Shabana