The primary difference between a tub seat and a shower seat is the seat height. Most units intended for tub use are 15 to 16 inches in height, while shower models are approximately 20 to 21 inches high.
The seat height for the tub model is intended to position the seat approximately level with the top of tub wall. Seat heights that are significantly lower than this often pose a problem for the user when it comes time to exit the tub. From these lower positions, body mechanics are bad for both user and attendant, creating the potential for a serious accident.