ASS styling is not understood by simple players, and it is lost when converting to plain formats. SBV imports directly into YouTube Studio. ASS is at home in anime and karaoke subtitles with fonts, colours and positioning; SBV is the usual choice for uploading and downloading YouTube captions.
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. YouTube's SBV has no styling and no positions, so the result holds the text and times only.