Unfortunately I don't think there's any specs available and the best information I can get about them is that they contain compressed pictures by
RLE
DVD sups are used by many authoring applications and can be demuxed by tools like pgcedit and ifoedit. Also just about every text sub/vobsub editor inputs dvd sups. When placed in a vob they can be decoded by many players without the use of an external decoder but I believe DirectVobSub can also decode these.
BD/HDDVD sups are gaining more and more software support. TSMuxer and eac3to demux subtitles to sup only. SupRip, BDSup2Sub, SupRead are sup editors/converters. TSMuxer/TSRemuX/TS4NP use them as input to mux to (m2)ts files. Only players I know of that can decode these are commercial BD players (PowerDVD, Showtime, WinDVD, Arcsoft, etc.) and MPC-HC which I believe uses
an updated vsfilter.dll.
The reason I ask for the sup is so original subtitles can be used without relying on OCR.