The "T" stands for temporary. It's like a trial period. After a certain length of time it will be changed to (P) for permanent if that position is handled correctly. The difference is the difficulty it would take to be taken away. In the Army, a temporary rank can be removed with an Article 15 (company level) A permanent rank usually takes a summery courts marshal. Perhaps the Navy has different rules for removal, but the definition of the (T) is correct.