A Xenos World is the designation given to a planet by the Adeptus Administratum that lies outside of the bounds and control of the Imperium of Man and that is the homeworld or colony world of an intelligent alien species. Most alien races inhabit only a single world that is their original homeworld, or a small group of worlds that they have recently settled in star systems close to their homeworld. The majority of intelligent alien species are comparatively technologically primitive, unaggressive or lack any true power in the interstellar arena, which means they are of no interest to the Imperium unless they are occupying unusually resource-rich worlds or other possess something the Imperium needs or represent a threat to the lives and property of Mankind. Only a few of the intelligent races in the galaxy are numerous enough, aggressive enough or powerful enough to serve as rivals of the Imperium who must be dealt with severely. Of these, by far the most common are the Orks, the Eldar, the Dark Eldar, the Necrons, the Tau and the Tyranids.
White Dwarf 139 (UK) states on page 9 that "Humanity is but one of many races in the galaxy. However, none are so widely distributed or so numerous as humans." Contradicting this, however, Codex: Orks (2nd Edition) on page 4 clearly states that the Orks are the most widespread intelligent race in the Milky Way Galaxy. It is possible in attempting to reconcile this information that the Greenskins are the more numerous race but are split into a multitude of different factions, whilst the Imperium of Man represents the largest single polity in the galaxy governed by a single species.
The Necrons are also now believed to be even more widespread across the galaxy than both Orks and humans. On page 17 of the Codex Necrons (5th Edition) it notes: "These Tomb Worlds represent only a handful of the many millions spread throughout the galaxy," while it continues on page 8 to state that: "even now, at the close of the 41st Millennium, billions of Necrons still slumber in their tombs, silently waiting the clarion call of destiny." This indicates that depending on the number of Tomb Worlds in the galaxy and the amount of Necrons sleeping beneath each, the Necrons could represent an even more widespread intelligent species that Mankind or their estwhile Greenskin foes.
- Codex: Necrons (5th Edition), pg. 17
- Codex: Orks (2nd Edition), pg. 4
- White Dwarf 139 (UK), "Spacefleet - Worlds of the Galaxy," by Jervis Johnson & Andy Jones, pg. 9