SCC is a hex dump that people cannot read or edit by hand, and web players do not read it. Plain text opens on every device and is easy to search or paste elsewhere. SCC is at home in closed captions for US and Canadian broadcast, DVD and some streaming deliveries; TXT is the usual choice for notes, logs and text that must be read by anything.
The text is read as UTF-8 or UTF-16, whichever the file is. An older file in another code page is read using the charset it declares, or as Windows-1252 if it declares none, so accented letters in an unlabelled legacy file can come out wrong. A file that is not text, such as the picture-based subtitles of a DVD, is refused instead of being converted to nonsense. CEA-608 pop-on, roll-up and paint-on captions on the first caption channel (or the second, if the first is empty) are read; colours are dropped, and a roll-up row becomes one cue that ends when it scrolls off. Times follow the 29.97 fps frame clock, drop-frame or not. The result is a transcript with one cue per line, without times; a cue's own line breaks become spaces.