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.
Also see Matroška.