A .sub file is one of three files that make up a CloneCD Image File. There should also be a .img file and a .ccd file. A .SUB file stores sub channel data on all tracks of the CD. It must be used with the .IMG file and the .CCD file to properly burn the contents to a CD.
These files must also be in the same directory (same folder on same hard disk drive) and must have the same file name. For example a folder like C:\Images\ would have to have IMAGE.CCD, IMAGE.IMG and IMAGE.SUB so that it can be burned to disc or mounted onto a virtual drive.
There is a variety of software titles that can make use of these image files. For example, Alcohol 120% or Daemon Tools can mount CloneCD Image Files onto virtual CD drives (so you don't need to burn to use them) or you can burn the image with CloneCD.
