diff --git a/pkgs/top-level/all-packages.nix b/pkgs/top-level/all-packages.nix
index 7a96521b05bf..2971461186a0 100644
--- a/pkgs/top-level/all-packages.nix
+++ b/pkgs/top-level/all-packages.nix
@@ -44,6 +44,7 @@ let
 
   # The contents of the configuration file found at $NIXPKGS_CONFIG or
   # $HOME/.nixpkgs/config.nix.
+  # for NIXOS (nixos-rebuild): use nixpkgs.config option
   config =
     let
       toPath = builtins.toPath;