What is the difference between what is referred to as a C-Model and a D-Model?  Technic sets often have an A & B model and I thought a C-model was a fan created alternate option, which leaves me wondering what D-model is.  

For instance, I'm pretty sure the Duck model I found online that is built using only parts from 42040 is considered a C-model.  (Fun model to build, though one of the 12 tooth gears doesn't like to stay put for long)

Some people call their 2nd C-model of a given set a D-model, just to differentiate it from the first. I don't think there's any technical difference - just down to individual builders.

For the C-model index, I just call them all C-models, as it doesn't make sense to start incrementing the letters when it's a collection of MOCs from different builders.

Community calls any alternate model consisting parts from a given set as "C-model". Some people produce more than one alternate model so they start calling them D, E... models.

Some older sets (e.g. 854, 856, first rev. of 8843) came with instructions for more than one alternative model, so those were also called "C-model" etc.