What is ASS?
ASS (Advanced SubStation Alpha) is the subtitle format for people who care where the text sits: styles, positioning, fonts, karaoke timing. Anime fansubs made it standard. Converting it to a plain format means the words survive and the choreography does not.
What is SRT?
SubRip subtitles are about as simple as a file format gets: a number, a start and end time, the lines of text, a blank line, repeat. Nearly every video player and editor on earth reads them.