Pre-requisite: Kin, Abhumans, Mutant
You live at the edge of the extremely narrow Imperial definition of "Human". You may be one of the many off-shoot strains of humanity, such as an Ogryn. You may have been afflicted by a visible Mutation due to inherited Corruption or exposure to baleful Warp energies. More likely, you've come down with a visibly chronic condition, such as jaundice, leprosy, or a bad case of eczema, and committed the cardinal sin of "not rich enough to hide it" (see also Navis Nobilite, Pellenne).
You may still be treated with suspicion, fear, or - worst of all - pity by your "betters", but you're usually afforded enough grace to eke out a living until the mob finds itself in need of a scapegoat. Though you are subject to the Stranger in a Strange Land rule, only half the normal penalties apply (i.e. -1SL to relevant tests involving Imperial people and equipment, and -1 Influence from relevant Factions). In addition, most specialisations restricted by this rule can be bypassed with purchase of a Forbidden Knowledge talent, to represent the extra hoops you have to jump through to get as far as other Imperial Citizens.