You can differentiate the PO's based on line type used. you may select the appropriate Line type in purchase order summary form and query the related the PO's
PO_LINES_ALL check PO_LINE_TYPES_B link and also link with PO_HEADERS_ALL
in table PO_LINES_ALL , you can filter PURCHASE_BASIS to select either GOODS or SERVICES.
you can link corresponding PO_HEADER_ID with PO_HEADERS_ALL to get related PO numbers
for requisitions just replace PO_LINES_ALL by PO_REQUISITION_LINES_ALL