Wait. What do chapters do? Are they like. Missions. Or are they like just gear?
Each chapter is like a separate mini-army of the Adeptus Astartes. Every chapter has their own unique history, homeworld, founder, traits, color scheme, etc. The Chaos equivalent is a legion also with their own histories, homeworlds, traits, colors, etc. They each have their own unique things, like Blood Ravens are one of the youngest chapters and have more psykers and are looked upon questionably sometimes (as a result of the history as explained in Dawn of War II games etc.), the Ultramarines are the most venerable and well known for being the god damned best etc.