What are the selection criteria used by APXIAWRE?

We have a customization in the APINVAPR workflow that causes an additional custom notification to be sent when approval is rejected. My understanding is that until this notification has been processed, it somehow prevents the invoice from being picked up by the scheduled Invoice Approval Workflow concurrent program APXIAWRE (it runs every night). So invoices for which approval was rejected will not be re-processed by the scheduled APXIAWRE until the notification has been dealt with. But for some reason, some invoices for which approval was rejected are being picked up by this request although the notification is still open.

In order to determine why that is, I need to understand how the selection criteria used by APXIAWRE, i.e. how is APXIAWRE prevented from picking up invoices where the custom notification has not been processed: is there something in APXIAWRE that checks whether any part of a previous workflow is still active (and excludes these invoices), or something along these lines?