[alreadyregistered]

Since no one else has yet to respond, I will venture an educated guess here. Since:
T = thumbnails
D = detail images
C = category images (i think)

I suspect that the others correlate to some module that you have turned off. For example, if you turn on the Manufacturer module (where you could use a manufacturer's logo with proudcts), the "M" folder will probably appear. Try it. It will cost you nothing!

XCart seems to create files and folders dynamically, on an as-needed basis.