About .SUB File Format
To work with SUB filesin Anycount, let’s get to know a little more about this format.
The SUB extension belongs to the MicroDVD subtitle format. Originally created for the MicroDVD Player program in 2000-2001, it is now one of the more widely supported DVD subtitle formats. It is extremely simple, using frame numbers rather than timecodes.
Link:
Aegisub (free SUB file editor)