The CA is configured to not store certificates and requests which satisfy certain criteria, setting the pending option would ...

The CA is configured to not store certificates and requests which satisfy certain criteria, setting the pending option would result in all requests being saved to the database irrespective of whether they satisfy the criteria of not being persisted to the database. The criteria to store certificates and requests depends upon a template setting for enterprise CAs and upon registry configured EKU OIDs for standalone CAs.