[Theli93]

MKS is just a Matroska containter for subtitles (only). It allows the inclusion of multiple subtitles, custom fonts, and the appropriate names/labels/etc without having to mux it all into the video. It works particularly well for ASS subs that make use of custom fonts. It works fine in VLC, but Kodi doesn't appear capable to even see the file type when manually searching.

I tried searching for something in advancedsettings, but found nothing for subtitle file types.