Originally Posted by Jersdawg

The contract says only if no FO reserves are available. I'll try to find the reference, standby.

Edit: section 12.G.a says: The assignment of Reserve Captains to First Officer flying will be done after all Reserve First Officers in domicile and all Ready Reserve First Officers in domicile are assigned to flights (as specified in Section 11.I.1.c.1-5) and are unable to complete the assignment.