At times when a mare has 'missed' or 'slipped' her pregnancy, you will choose to cease the mare's breeding service that season.

In Prism, you can click the Abort action in the Mare Profile > Breeding screen, which will allow you to specify the mare's last known status (e.g. missed or slipped) and remove any and all pending breeding tasks linked to that season's stallion booking.

Now, when you choose the Abort action, a completed task is added to the booking for improved traceability.