A configurable Subsystem is a kind of block that is made available as a standard part of Simulink. In effect, a configurable Subsystem is a block for which you can specify several underlying blocks. Each underlying block is a possible implementation, and you are free to choose which implementation to use. In System Generator you might, for example, specify a general-purpose FIR filter as a configurable Subsystem whose underlying blocks are specific FIR filters. Some of the underlying filters might be fast but require much hardware, while others are slow but require less hardware. Switching the choice of the underlying filter allows you to perform experiments that trade hardware cost against speed.