From Speedsolving.com Wiki
A subset is a group of algorithms, typically all of which within that set have something in common, which can collectively be used to solve all cases of a certain type.
Examples of subsets include:
- PLL (preserves OLL and F2L but affects permutation of the last layer)
- OLL (preserves F2L but affects orientation of the last layer)
- CMLL (preserves F2B but affects the top layer's corners)
- ZBLL (preserves F2L and EO but affects last layer permutation and corner orientation)