8de00c328a
"require" is a stronger version of "want", and just like wantedBy allows you to specify this relation in reverse, requiredBy does the same. It may seem pointless to be able to specify these stronger relations in reverse, because if something is really required, you would expect the other unit to specify this himself. However, this is still useful for virtual/automatic units (like devices) that are created by systemd on demand and hence have no unit file you can alter.
*** 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.