SUB is an extension given to files containing the text of subtitles used during movie projection. These are mostly dialogue lists that are translated from the original language to the target language. This solution is also used for the needs of people with disabilities, e.g. the hearing impaired. A SUB is a text file which, in addition to the main content, also contains information at which point in the movie a given fragment of the text is to be displayed.

Often, SUB files are associated with software called CaSat Suite where you can create dialogue lists for movies.