Could you direct me to some manual or description of what exactly joint armature does and in what units it is measured? Thank you.
This value is added to the diagonal of the joint-space inertia matrix. It has the same units as inertia. Physically, it corresponds to the rotating part of a motor -- which increases the inertia of the joint, even though the rigid bodies forming the joint can have very little inertia. More generally, it makes the dynamics more stable and easier to simulate at large time steps.