<nixpkgs>/modules
This directory hosts subdirectories representing each module class for which the nixpkgs repository has user-importable modules.
Exceptions:
_class = "nixos";modules go in the<nixpkgs>/nixos/modulestree- modules whose only purpose is to test code in this repository
The emphasis is on importable modules, i.e. ones that aren't inherent to and built into the Module System application.