No official documentation.
There have been a couple of projects which have written out .c2d files:
- Rabbeted box generator for Carbide Create
- Cutter Pack Giveaway! - #11 by Julien
- Facebook Groups
- Ruler in Carbide create - #33 by ClayJar
- Combining 2 c2d files? - #4 by FrankBuss
The problem is, the file format has been something of a moving target — keeps changing — not sure when the file format will stabilize.