Aptus Non, also sometimes referred to as Adeptus Non, is a Tithe Grade assigned to worlds of the Imperium of Man by the Adeptus Administratum. It is a High Gothic term which translates into Low Gothic as "Not Applicable." The assignment of the grade of Aptus Non to an Imperial planet means that its population owes no tithe to the Administratum. This is because most of the worlds assigned this grade already provide a vital service for the Imperium, such as Adeptus Mechanicus Forge Worlds or Adeptus Astartes Chapter homeworlds, or because they have nothing of value to offer, such as lifeless Dead Worlds.
However, Space Marine Chapter planets are expected to provide one form of tithe to the Imperium -- 5% of the Chapter's gene-seed given to the Mechanicus. The Mechanicus maintains these stores on Terra in the event that should the Chapter suffer from rampant mutation, Chaos corruption or simply be exterminated in the course of its duties, it can be refounded or contribute to the raising of a Successor Chapter.
- Warhammer 40,000: Rulebook (3rd Edition), pp. 114-115