This section describes how to add additional packages to your system. NixOS has two distinct styles of package management: Declarative, where you declare what packages you want in your configuration.nix. Every time you run nixos-rebuild, NixOS will ensure that you get a consistent set of binaries corresponding to your specification. Ad hoc, where you install, upgrade and uninstall packages via the nix-env command. This style allows mixing packages from different Nixpkgs versions. It’s the only choice for non-root users.