Chapter Serf
Space Marines are big tough supermen and all, but even they need a helping hand. Someone has to to polish their armor and nob, 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.
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 (and were deemed able and worthy) to serve. The children of Chapter Serfs also often become chapter serfs themselves. 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 Marine Bolters. Not out of elitist exclusivity or anything, but rather because they are heavy and far too unwieldy for a regular human to use to any degree of real effect as a weapon. Heck, it's probably logistics: it's unnecessary busywork managing Marine pattern bolters the serfs can't use, as well as the Sisters/Guard pattern bolters that are a bit too small for Marines and use different size rounds and mags.
How well they're treated depends on the Chapter they serve. Ultramarines, Blood Angels, Space Wolves, and Salamanders treat them as equals to other Space Marines, like soldiers might treat their friends in the supply corps. The Wolves use their serfs as a defence force on Fenris, known as "Kaerls", though we don't know if they get to go on campaign or not except for those serving in the fleet. The Star Phantoms entrust them with maintaining the chapter's faith and up keeping their massive ossuary. Blood Drinkers, being vampires, let them volunteer to be ritually drained of blood for them to (surprise surprise) drink to stave off the Red Thirst. Iron Hands and Imperial Fists and their countless successor chapters see them as just kind of there and don't really care one way or another, the way you treat the fast food workers and garbage people who make your comfy life possible. Marines Malevolent, being dicks, treat them like slaves. Flesh Tearers being psychotic, treat them like stress balls to work out frustration on between missions. (Spoiler alert: a human can't usually survive being used as a stress-ball by a Marine.)
Don't even get us started on how the Legion Serfs in the Traitor Legions are treated (like really bad, imagine being forced to watch erotica berzerker porn with bloody panties and tampons. Don't even think about it! Some are treated well. Talos of the Night Lords treats his serfs very well.) More often though, Chaos Warbands doesn't have actual Serfs, but rather use Cultists, who are pulled towards the Warband to worship them. They are very often treated like absolute shit by the Traitors, since their own hubris and newfound freedom doesn't go well with 'being nice to people below you'. If nothing else, most Chaos Lords knows how important a healthy supply of fanatic cults is for the runnings of a Warband.
Of the various traitor legion depictions so far, the Emperor's Children post-heresy mistreated their "serfs" to death and had to resort to stealing everyone else's during the Great Scouring (causing a huge civil war amongst the traitor legions), one band of Iron Warriors used forced slaves instead of chapter serfs (and they ended up being just as reliable as you'd expect when the shit hit the fan), the Thousand Sons use a combination of Tzaangors (whom are actually treated relatively well and respected) and enslaved native human population from the Planet of Sorcerors (whom, should they manage to escape and survive, become primary candidates for new Thousand Son marines. That's how Tzeentch rolls), and the Crimson Slaughter has an entire city's worth of them in the depths of their battle barge that they mostly ignore (they tag alongside the Crimson Slaughter in battle hoping to get noticed, and those that do become new Crimson Slaughter marines).
In summary they are the Alfred of 40k, because no fine super hero are without a friend/allies that would comfort them after all the grimdark shit happened outside to fight another day.
Rules for Serfs
Chapter Serfs are represented with various rules both in the 40kRPGs and on the tabletop:
On the tabletop (both in 40k by the Badab War books, and in 30k Extermination) they are represented in battles where space marine capital ships get invaded by enemy forces, where the crew has to get involved by necessity. They come in squads of 6, are armed with either laspistol & CCW or Shotguns and wear void-hardened flak armour. However they have the same stat-lines as Imperial Guard Conscripts because they aren't battle-hardened fighters, or expected to be.
The Deathwatch (RPG) does it very similarly, having stat lines weaker than starting Dark Heresy player characters, and if you take the tens digit of their statline it very nearly exactly follows the tabletop stats for them too. What is different about them though is that chapter serfs do have biological enhancements, and have the Unnatural Strength, Toughness and Senses traits, as well as Dark Sight. Which actually falls into line with how the main character of Prospero Burns is presented. (though he's not strictly a Serf, his enhancements were apparently no great expense for the Legion)
However, with the ally rules, you could probably take them as Imperial Guardsmen, just modeled differently. What to do with all those Spare Marine and scout torsos? Give em lasguns and play them as Chapter Serfs.
Serfs' Role
Granted, even serfs are specialized in specific roles and not an all-rounder 'slave'. Here are the most common roles that serfs might serve in a Chapter
- Artificers - Technicians and assistants to Techmarines. Routinely polishes and conducts minor repairs on their master's power armor and wargear while the Techmarines handle the more complicated tasks.
- Master of the Refectorium - These are the 'head chef' of a Chapter's kitchen, ensuring the hefty amount of food served is prepared at its highest quality. Some of the most prestigious position that a serf might get into since even Space Marines would love to have a well-served meal.
- Medicae - Nurses to the Apothecarys' doctor. As the Space Marines' anatomy is complex beyond comprehension, these are often staffed by the most intelligent and dexterous serfs.
- Ordinator of the House - Chapter Master's personal teddy bear. Well performed serfs (usually on the field of battle) can catch the attention of their Chapter Master lords and get promoted to this position. Acting as the nominal head of the Serfs, these folks ensure that other Serfs are smoothly performing their duties and get the special privilege of being in the presence of the Chapter Master at any time.
- Monitors - Messengers, engineered to be able to remember pages long worth of information and SOMEHOW ENCRYPT THE TEXT and DECRYPT it when given correct verbal code. Fleshy 1940s Enigma Machines, creepy
- Scribellum - Clerks to the Chapter's Librarians. Chronicle the Chapter's history and deeds, also assists in creating Purity Seals.
- Sacratium - Lay assistants to Chapter's Chaplains. Tends Chapter's relics and assists in perform ceremonious tasks.