Reference frame of GetDenseJacobian?

I'm trying to model a mobile manipulator consisting of a base (3 DOF) and a manipulator (6 DOF). For control I need the Jacobian. I noticed the ArticulationBody and GetDenseJacobian, which I'm trying to use to avoid calculating the Jacobian by hand. I've set target velocities to the root ArticulationBody to control motion of the base, with zeroes in the directions I'm not supposed to be able to control.

My question is: in what reference frame is the obtained Jacobian expressed?

I assume multiplying the Jacobian* by joint velocities results in the velocity of the final ArticulationBody in the tree, but I haven't been able to figure out in what frame this is expressed. I would like this to be expressed in the root of the ArticulationBody, but that does not seem to be the case.

* I spent some time trying to figure out how to interpret this, my current understanding is that each set of six rows represents one ArticulationBody in the tree starting from the root. If so, the final six rows would be for the final ArticulationBody in the tree. The ArticulationBody used with GetDenseJacobian does not seem to impact the values, so now I'm using the root.

I'm on Unity 2020.3.1f1.