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A format used to hold bitmapped subtitles for films on DVD disks. After ripping a DVD film to disk using something like makemkv, the subtitle track can be extracted to the VobSub format (a .idx / .sub pair of files) using mkvtoolnix.

I haven't used this myself but there's a Rust library for the VobSub format at Rust VobSub with information about the format. The binary .sub data is apparently a MPEG program stream wrapping a Packetized Elementary Stream wrapping run length encoded image data.