See https://nixos.wiki/wiki/Creating_a_NixOS_live_CD for a little flavor

The

> -I nixos-config=iso.nix

in

> nix-build '<nixpkgs/nixos>' -A config.system.build.isoImage -I nixos-config=iso.nix

is basically saying "use this file instead of my regular `/etc/nixos/configuration.nix` entry point".

The format of the two however is the same.