ASS styling is not understood by simple players, and it is lost when converting to plain formats. CSV is plain text that every spreadsheet, database and script can read. ASS is at home in anime and karaoke subtitles with fonts, colours and positioning; CSV is the usual choice for moving tabular data between systems.
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. Each line takes its italic, bold, underline and alignment from its style and from override tags; drawing commands and karaoke effects are ignored. The result has the columns index, start, end and text, with times as HH:MM:SS.mmm, and starts with a byte-order mark so Excel reads accented letters correctly.