“DoF and rotation limits are defined with respect to the rest position of the bone” for bones in IK chains. I take this to mean that if you set Lock Rot for all 3 axies of a bone it would remain in it’s rest postion relative to its parents once you enter pose mode.
However, sometimes a bone with all axies locked takes a completely new rotation (in pose mode) which seems to have no relation to it’s rest postion.