The 2 tables can be joined in the following manner:
SELECT AP.VOUCHERNO, AP.PONO, AP.VENDOR_CODE, W.*
FROM PPA_WIPAPINFO AP, PPA_WIP W
WHERE AP.ID = W.PPA_WIPAPINFO_ID
The PPA_WIPAPINFO.PONO number is from XOG or Voucher Transaction Entry transactions (not used in Timesheet transactions)
The PPA_WIP.INVOICE number is generated from the use of chargeback invoices.