For the joint, what does the numerical value of frictionloss and damping actually mean? Do they mean the coefficients? Is there an upper bound for these values? And in urdf files, there are also 'damping' and 'friction' tags for joints (e.g. https://github.com/RethinkRobotics/..._description/urdf/sawyer_base.urdf.xacro#L174) , do they correspond to 'damping' and 'frictionloss' here in mujoco for joints? Thanks.
Frictionloss is the amount of force or torque that is lost to friction. This refers to load-independent friction. The math is described in the Computation chapter in the documentation. Damping is the usual damping coefficient. These values should be non-negative. There is no upper limit, but if you make them too large the simulation can go unstable (unless you use very small time steps). They indeed correspond to 'damping' and 'friction' in URDF.