This enlarges the system uid/gid range 6-fold, from 100 to 600 ids. This is a preventative measure against running out of dynamically allocated ids for NixOS services with isSystemUser, which should become the preferred way of allocating uids for non-real users.
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.