In my understanding, pre_mul, user_mul and cam_mul were mutually exclusive:
if use_camera_wb=1, cam_mul was being used, if use_camera_wb=0, it was pre_mul, and if pre_mul was defined, then pre_mul was being used instead of the 2 others.
However I ran the following test, showing that it is not true:
no_auto_bright = 1
In [...], I try to use "as shot" white balance, setting either user_mul, pre_mul or use_camera_wb=1.
I was expecting all 3 pictures to be identical... but they're all different!
Here are the images.