For uploaded imports: interwiki prefix to apply to unknown user names (and known users if $1assignknownusers is set).
You shouldn't even be seeing this. Bizzeebeever told me to add this here so that we can get responsiveness working for all screens.
From The Volumes of Truth
For uploaded imports: interwiki prefix to apply to unknown user names (and known users if $1assignknownusers is set).