Subs (as in a shortening of subhuman), aka Homo sapiens deformum, are the degenerate strains of Abhuman found throughout the deepest pits of Humanity. They evolved after millennia of living in the nastiest shit imaginable and basically resemble the Ghouls from Fallout.
Subs are sometimes confused as Mutants, but they possess a consistent morphology to be classified as a human subspecies. Nevertheless, your average Imperial citizen are unable to differentiate the different factions of Elfdar, let alone guys who most definitely skirt the line fugly.
Seriously, they are often described as having skin resembling rusting metal, heads sunk so low the face peers from the chest, limbs that writhe like serpents, eyes that radiate dazzling light and so on. You sure they ain't Chaos Mutants?
As such, they are often frequent victims of Ecclesiarchy purges, forcing them to live a life of bullshittery.
Despite this, several groups are still known to be loyal to the Imperium and seek redemption in battle like Beastmen. They are often useful despite their appearance, utilizing mutations that have practical uses on worlds with harsh local conditions. These Subs are known to be zealous warriors, and are far more numerous than the High Lords of Terra realize.