There are two kinds of invoices that can be processed by Data Archive - Simple invoices that are not matched to a PO and invoices that are matched to a PO. The Matched PO or Invoices entity processes both kinds of invoices (matched and simple), since it is possible that simple and matched invoices can be part of the same transaction chain through checks, POs, and requisitions. This rule ensures that all links in the transaction chain for a given invoice are purgeable.