A recent question on
https://www.mediawiki.org/wiki/Extension_talk:User_Merge_and_Delete#Usernam…
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?