Unfree software
Unfree software refers to software that has restrictive licensing on modification and/or redistribution. This type of software cannot be freely provided or distributed in an official capacity, which means that unfree software is neither built by Hydra, nor cached on the official binary cache. Despite this, Nixpkgs offers a very large collection of unfree software as derivations, however they cannot be used by default without configuring Nixpkgs and opting in to unfree software usage.