what does the"rpy" in URDF file means?

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Some hints after doing this recently:

Roll is applied first about the X-axis of the previous joint. The frame you're modifying will rotate X radians about the previous X-axis.

Pitch is applied next about the Y-axis of the previous joint. Yaw is applied last about the Z-axis of the previous joint.

Each rotation seems to be capped at pi radians. For example, if you enter a pitch of 3.9 radians, the actual rotation you see will only be pi radians.