Unlike standard subtitles that only show plain text, SSA files include data for: SSA File Extension - What is .ssa and how to open?
.ssa (SubStation Alpha) video is a type of video file that contains subtitles or closed captions. The .ssa file extension is commonly used for subtitle files that are used in conjunction with video files, but it can also be used to store video content with embedded subtitles. .ssa video
Most modern players (VLC, MPC-HC, PotPlayer) support both, but for new projects. Unlike standard subtitles that only show plain text,