From Halopedia, the Halo wiki
Crewman Apprentice, abbreviated to CA, is a enlisted rank in the United Nations Space Command Naval Command (NAVCOM) rank structure; including the UNSC Navy and Office of Naval Intelligence. It is preceded by Crewman Recruit, and subordinate to a Crewman.
Crewman Apprentice are given an initial fleet assignment after the graduate boot camp and training school. A possible assignment Crewmen Apprentice can get during the assignment is atmospheric reclamation maintenance duty.
Duration start and end dates are given based on the earliest and latest information possible.