Rear Derailleur have 3 type in Long, Med & Short Cage.

Normally Cross Country Trail Rider use 11-34T Cassette & 44-32-22T Triple Crankset. That will require a Long Cage (SGS) Rear Derailleur.

Double Chainring with Bash Guard 32-22T, will fit for Med Cage (GS)

Riders using single chainring & 11-34T cassette can run with a Med (GS). Downhill rider so often take a Short Cage (SS) road rear derailleur, because they don't climb, they want the extra clearance of a more compact derailleur.