Difference between C & D models?

[teos]

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)

[Josephiah]

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.

[zux]

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.

[Carrera124]

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.