Manual:$wgUserrightsInterwikiDelimiter

Details
Character used as a delimiter when testing for interwiki userrights (In Special:UserRights, it is possible to modify users on different databases if the delimiter is used, e.g. Someuser@enwiki).

It is recommended that you have this delimiter in $wgInvalidUsernameCharacters above, or you will not be able to modify the user rights of those users via Special:UserRights.