Variant Subsystems are types of blocks that are available as a standard part of Simulink. You can configure variant Subsystems to specify several underlying blocks.
Each underlying block is a possible implementation, and you are free to choose which implementation to use. In Vitis Model Composer you might, for example, specify a general-purpose FIR filter as a variant Subsystem whose underlying blocks are specific FIR filters. Some of the underlying filters might be fast but can require significant hardware resources, 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.