What's the difference between DEF bones and ORG bones in functionality? I have downloaded a very nice, fully rigged, character model and have embarked on studying this file and doing some tests. However, from the theoretical POV, I can't seem to grasp the difference between DEF bones and ORG bones in that file. The file is "Mr Incredible" from Blendswap, all credits go to "Crias" the author of that file, not sure if I am allowed to re-upload his file here.

But in theory, how does each bone function and contribute to the rig? considering that a MCH bone is for mechanism, DEF for deformation, CTRL for Control..etc, I'm aware of the terms and functions for each type, except ORG, don't know if it's a sub-type of DEF or I got it completely wrong.

EDIT: Sure, here's a screenshot of the selected ORG bone:



It lies within another bone named "Shoulder.L" and this bone does deform the body upon rotation:
ORG - prefix for original bones from which the full rig is generated

DEF - prefix for bones contributing to deformation

Usually the ORG bones of the metarig are replaced by generated bones, but in this case the clavicle is used from the original rig and contributing to deformations.

This IMHO is a mistake and should be marked right, a copy of that ORG bone should be generated and marked DEF and the ORG bone moved to a hidden layer where rest of the org bones are lurking, ready to be tweaked and a new full rig generated from them again if needed. If both are the same bone, you do deform the mesh while tweaking the metarig and this is not wanted, the mesh should stay intact. If the org bones are in a separate armature (like with rigify), there is no need to have ORG prefixed bones inside the rig at all.