Sub Station Alpha

SubStation Alpha, abbreviated SSA or ASS, is a subtitle file format that allows for more advanced subtitles than the conventional SRT and similar formats.

This subtitle format is frequently used in anime fansubs due to it's capability, either to overlay subtitles onto video while it is being encoded (hardsubbing), or to store subtitle data alongside video data, often in a Matroska (MKV) container (softsubbing). It's not commonly used professionally except by Crunchyroll.

The most common open source subtitling application that support the SSA/ASS format is Aegisub.