Questions on Observation Structure in AMP Paper Implementation

[Ladun]

Hello. I am a student currently implementing the AMP paper myself. I have a few questions about the structure of the observations described in Sections 5.3 and 6.1 of the paper.

1. The paper separately describes the speed and position of joints and links. How are joints and links different from each other?
2. In Section 5.3, does the "Local velocity of each joint" refer only to linear velocity, or does it also include angular velocity?
3. In Section 5.3, how is the local angle for revolute joints handled?

Thank you in advance for your help.