Hello @Wataru Thanks for reaching out on this forum.
As of now, the below documentation is the source of definitions related to the Azure Kinect DK. Please let me know if that helps?
Azure Kinect body tracking joints
Also, See references where we are having active discussions on this topic:
https://learn.microsoft.com/en-us/answers/questions/542609/joint-points-in-azure-kinect-dk.html
https://github.com/microsoft/Azure-Kinect-Sensor-SDK/issues/1414