.SU refers to the official domain extension or the internet country code top level domain (ccTLD) of Soviet Union. It was introduced in 1990 and was intended for entities with connection to the USSR. Russian Institute for Public Networks is the authorized unit for carrying out the registry for .SU domains.
There is no special process needed to register a .SU domain name, no special action is needed on your end.
Used by entities connected with post-Soviet states