The description of utf8 format expansion
A utf8 file is a text document that uses Unicode UTF-8 (8-bit Unicode transformation Format) encoding and can be used for English and many other languages, including support for Asian characters. The utf8 format is compatible backwards with ASCII. As far as the formatting of the document is absolutely plain, there’s no need for any special text editor in order to open it.
It is worth to note, that the utf8 files may contain text-based subtitles, which are used during the video-playback.