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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.
Related software tools
Very powerful CD, DVD and CD/DVD image file manipulator/data recovery tool