ASS files carry fonts, colours, effects and drawing commands that only capable players render. A TV, a phone or a web upload often shows nothing for them, or shows the raw style tags. SRT keeps the words and timing and works everywhere, at the cost of the styling.
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. Of the styling, only italic, bold, underline and the position on screen (top, middle or bottom, left, centre or right) travel between formats; fonts, colours, outlines and effects do not. Each line takes its italic, bold, underline and alignment from its style and from override tags; drawing commands and karaoke effects are ignored. Positioned lines are written with {\an} codes, which many players honour; players that do not simply show them at the bottom.