The Cantle bolster, is a correction pad you put between the base and the seat, on the backside, to make the seat smaller.
The correction pad is particularly useful if there are several riders on 1 horse, and where there is a difference in your pant sizes and – or shape. If the saddle is bought for a person who wears a size 42 in trousers and has to share a saddle with a person who wears a size 34 in trousers, then the person who wears a size 34 in trousers can insert the bolster when they ride, to get the optimal support during the ride. When the rider who uses a size 42 has to ride, the bolster is taken out again.
You can se how to place your cantle bolster here