Chapter Serf
Space Marines are big tough supermen and all, but even they need a helping hand. Someone has to to polish their armor, cook their food, sweep their halls, clean their toilets, file their tax returns, and keep the fortress-monastery defended when the brothers are out stomping Bloodletter faces into the ground and tearing Ork rectums apart with chainswords. These people are Chapter Serfs and live comfortable, well-to-do lives... at least by Imperium of Man standards.
Although overall, how well they're treat depends on the Chapter they serve. Ultramarines and Salamanders treat then as equals to Space Marines. Imperial Fist and there Successor Chapters see them as just there and don't really care. Marines Malevolent being dicks treat them like slaves.
Chapter Serfs (formerly Legion Serfs), were originally picked from failed Space Marine Aspirants and other natives of Space Marine homeworlds who, though physically unable to become Space Marines, still wished to serve. Generally speaking, Chapter Serfs are well-trained, highly educated, and treated well; they are officially part of the Chapter. They also have access to highly-sophisticated weaponry (even the Imperial Guard don't get access to this shit). Of course, that does not include Bolters, not to be exclusive or anything, but rather that they are heavy and if a mere mortal managed to pull the trigger on one of them it would snap their Humerus like a slim jim.