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 stopping Bloodletter faces into the ground and fighting Orks. These people are Chapter Serfs and are generally well treated and live comfortable lives, at least by Imperium of Man standards.
Chapter Serfs, originally Legion Serfs, were originally formed out of failed Space Marine Aspirants and natives of the Space Marines homeworlds who were physically unable to become Space Marines but still wished to serve. Generally, Chapter Serfs are well trained, highly educated (by Imperial standards) and treated well by most chapters, and are officially part of the Chapter. They also have access to weaponry of much higher sophistication that the Imperial Guard doesn't even get (though not Bolters, though that's more because they can't lift them).