What Is A Sub File?

[sparsely]

Paul is correct. A .sub file is most likely part of a CloneCD image set. To burn it you need the other
files of that set. There's a .ccd, an .img. and a .sub.

[toddiscool]

just to add a little note the .ccd file is just like a .cue - meaning these are easily created. I am not sure on the exact way to do it but there are instructions in the help menu of clone CD.