A recent question on https://www.mediawiki.org/wiki/Extension_talk:User_Merge_and_Delete#Username... pointed me to that "Usernames with "@" characters" issue.
The following pages are known to me (Tim-SVG, thanks for answering on the extension's page)
+ https://www.mediawiki.org/wiki/Manual:$wgUserrightsInterwikiDelimiter + https://www.mediawiki.org/wiki/Manual:$wgInvalidUsernameCharacters
Can anyone explain to me or point me to a documentation page what the purpose of "@" in Wiki usernames is?