GMB1 and GMB2 messages

Hi,

I don’t really understand the meaning of GMB1 and GMB2 messages in .tlog files.
I found an explanation at this link: Onboard Message Log Messages — Copter documentation , but it is not clear to me.

GMB1:
What is the meaning of “delta-angle sum”, “delta-velocity sum” and “joint angle”?
“delta-angle sum”: is it the integration of the angle velocity in “dt”?
“delta-velocity sum”: is it the integration of the acceleration (acceleration or angular acceleration) in “dt”?
“joint angle”: is it the measured angle of the gimbal measured by motor encoders or by gimbal IMU? what is the joint?

GMB2:
Gimbal EKF estimate of gimbal angle: is it the estimate of the gimbal angle with respect to NED or another inertial system? Or the angle of gimbal with respect to drone frame?