Subtitles on live stream
There are several ways to transfer these subtitles: picture or text. Text requires device to render with some font in proper place on screen, picture goes with prerendered text.
Most subtitles from satellite are going in picture and ffmpeg identify this track as dvb_subtitle.
Text subtitles from satellite are identified as dvb_teletext. HLS has another format of such subtitles and it is called webvtt.