In nixos/nixpkgs sourec you'll find <nixpgks> and <nixos> like
placeholders. They are defined by NIX_PATH="nixpkgs=foo:nixos=bar".
The reason is that in the past /etc/nixos/{nixpkgs,nixos} has been the
source repostiories to build up nixos (This may have changed - I'm not
up to date).
Marc Weber