Stovall, Joel Clinton
At the end of every quarter (3-31, 6-30, 9-30, 12-31), an entry was made to record the sailor's current assignment. (This may have been abandoned after 1942-06-30.) Other entries record the date and type of a "Change of Status", sometimes including the "Cause and Character of Discharge". A status change of type "T" indicates a transfer out - i.e. the last date aboard the "Ship or Station". (After 1942-06-30, "Rec" was added to indicate the first date on board, maybe to allow travel time from the previous "T".) A status change of type "CR" indicates a change of rate - typically a promotion.
I believe that the long assignments to the hospital are actually periods of leave, which started and ended with clearance through the hospital.