The Bluray m2ts format enables 2 types of on-screen overlays that can be used for subtitles. Is based on text but concerning now I've seen no BluRay by using one for subs. Another is the PGS (Presentation Grapic Stream) and consists in bitmaps (and also the timeframes what is the best weather resistant show up). That second stream is by far probably the most popular by BluRays. As we'll see below, tools exist to extract that stream to .sup files. It's not exactly the same format as .sup files that some tools extract from DVDs.
Be aware that HD DVD have likewise a .sup format which is slightly not the same as the BluRay one. As far as the PCH is just not (yet) able to display PGS, the only way to get subs for ts or m2ts material is to apply a side document (.srt) containing the subs.