From Halopedia, the Halo wiki
Due to Recluse being a classified armor, a lack of ethical, legal and administrative oversight would allow hypothetical RECLUSE designers to fully exploit Spartan neurological peculiarities - albeit at great risk of inducing dangerous side effects.
The RECLUSE does not appear in any UNSC logistics tracking system, nor does it bare any manufacturing markings. It is a suit that doesn't exist, created for missions that never happened, by Spartans who never were.
Halo 5: Guardians
Internally, Recluse was known during development by the codename "A055".
The Recluse armour may be named after the venomous Recluse spider, with the Laeta and Seri skins named after the subspecies "Loxosceles laeta" and "Loxosceles seri" respectively.
List of appearances