CLOSED_CODE in Table: PO_REQUISITION_HEADERS_ALL

I was asked to find all "OPEN" requisitions in my company. I noticed in requisition header table PO_REQUISITION_HEADERS_ALL, the CLOSED_CODE column can be NULL, "OPEN" and "FINALLY CLOSED". What does NULL value for CLOSED_CODE mean? Is the requisition considered open if its CLOSED_CODE is null? what is the difference between NULL and "OPEN" for CLOSED_CODE column ?

Thanks