CESM's component models can be run in a variety of combinations, ranging from all components being active to all components being off. A given set of component configurations is known as a "component set" or "compset" for short.
Similar compsets are grouped using aliases. The CESM2 documentation provides a table showing which components are active.
Coupled data assimilation experiments typically use compsets with aliases
beginning with B, since the atmosphere, land, sea ice and ocean components
are all active in these compsets.
The CESM website provides a comprehensive list of all available compsets.