From Lexicanum

Aptus Non (also referred to as "Adeptus Non") is a tithe grade used by the Imperium's Administratum, meaning not applicable.

The Aptus Non tithe grade means that no tithe is required to be paid. Most classes of Aptus Non worlds either provide service to the Imperium in a unique way (Adeptus Mechanicus forge-worlds, Adeptus Astartes "Chapter Worlds") or are worlds which would simply not be taxed, such as lifeless dead worlds, and worlds unable to produce or provide anything of value.

However, Space Marine Chapters have to give 5% of their gene-seed as a "tithe", for various purposes. It is imperative that the gene-seed is examined for signs of taint or mutation, and if the particular chapter is wiped out, a portion of their gene-seed remains, ensuring that one day the chapter might be re-founded.

Sources

• ¹: Warhammer 40,000 3rd Edition Rulebook, pp. 114–115