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. After all, Space Marines are knights in spaaaace! and every knight needs his retainer. 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, and therefore get access to the fancy toys in the space marine armoury.
How well they're treated depends on the Chapter they serve. Ultramarines, Blood Angels, Space Wolves, and Salamanders are well aware they couldn't purge aliens, mutants and heretics without the 'unglorious' support Serfs provide and 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. They don't really care one way or another and form little bonds with them, considering service to be its own reward (think 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.)
As for the the Traitor Legions, it varies from "tolerably bad" to "absolute hell", mostly depending on the guy in charge and what God(s) he worships. Most often though, Chaos Warbands doesn't have actual Serfs anymore but use Cultists who are pulled towards the Warband to worship them as living avatars of the Chaos gods. Cultists are most of the time treated like crap by the Traitors, since their own hubris and newfound freedom doesn't go well with 'being nice to people below you'. But on the other hand, Chaos Lords knows how important a healthy supply of fanatic cults is for the runnings of a Warband and the value of meatshields in combat. Even bands of Khornate Berzerkers still have mortals tagging along with them despite their constant urge to kill anything crossing their path in the name of the Blood God, which means they do show some restraint. (of course, said mortals are just as bloodthirsty and just as often end up killing some of their own, but Khorne cares not from where the blood flows).
Of the various Traitor Legions depictions so far, the Emperor's Children are the worst, mistreating their "serfs" to death so hard they had to resort to stealing everyone else's during the Great Scouring (causing a huge civil war amongst the Traitor Legions). The Word Bearers mostly see mortals as a reserve of walking sacrifices waiting to happen. One band of Iron Warriors used slaves instead of chapter serfs, seeing them as nothing but replaceable cogs in their machines of war (and they ended up being just as reliable as you'd expect when the shit hit the fan); Honsou however sees the armies of cultists in his service as low-quality troops that nevertheless have their use so he values them a tiny little bit. The Thousand Sons use a combination of Tzaangors (who are actually voluntary employees who are treated relatively well and respected) and enslaved native human population from the Sortiarius (who, should they manage to escape and survive, become primary candidates for new Thousand Son marines. That's how Tzeentch rolls). The Crimson Slaughter has an entire city's worth of serfs in the depths of their battle barge that they just ignore most of the time (they tag alongside the Crimson Slaughter in battle hoping to get noticed, and those that somehow distinguish themselves get a shot at becoming new Crimson Slaughter marines). Talos Valcoran of the Night Lords treats his serfs surprisingly well (better than even some Loyalist Marines), and the others in his warband at worst see mortals as useful tools that shouldn't be wasted pointlessly.
Alpha Legion warbands operate at the halfway point, maintaining unaugmented operatives who, depending on the mission, stand a roughly fifty-fifty chance of being trusted with roles vastly more important than the Astartes themselves or being sacrificed to the enemy in interference action. Like the loyalists, these are considered members of the chapter. Like most other traitors, that makes then far from indispensable.
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 and melee weapon 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 possibly because instead of giving him a augmetic eye they just rammed a wolf eye in there, but they replaced every organ system but his nerves with new ones).
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.
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
- Sacratium - Lay assistants to Chapter's Chaplains. Tends Chapter's relics and assists in perform ceremonious tasks.
- Scribellum - Clerks to the Chapter's Librarians. Chronicle the Chapter's history and deeds, also assists in creating Purity Seals.
- Shipmaster - The commanding officer of a ship in the Chapter's fleet -- they would be the Captain if that wasn't their boss' rank. They oversee the ship's operations, including combat when the Space Marines are engaged in fighting off opponents.