The relationship among the mass (and some other) values in <rigid_body>, <instance_rigid_body>, and <shape> are sort of glossed over. Should be mentioned in more places.
1. In <inst_rig_body>:
a. in Child elements for technique common section, add this to intro before table:
Any values specified in elements within the instance override the values for the same elements in the referenced <rigid_body>.
b. Add section after child elements titled "Density, Mass, and Inertia Specification Rules". Content is just:
Refer to <rigid_body> for details.
2. In <rigid_body> "Density, Mass, and Inertia" discussion:
a. The final bullet starting "both the..." should not be a bullet.
b. Add two more paragraphs:
NOTE: <instance_rigid_body> allows any or all child elements of <technique_common> to be overridden, or to be specified in the instance if they are not specified in the original <rigid_body>. In this case, the same preceding rules apply, using any available values from <instance_rigid_body> in place of the <rigid_body> values.
Initial angular and linear velocity for a rigid_body can be specified in <instance_rigid_body>.
3. In <shape>, add another paragraph to the Concepts:
NOTE: The values for <hollow>, <mass>, and <density> are ignored if the parent rigid body specifies its own mass and inertia. See "Density, Mass, and Inertia Specification Rules" in <rigid_body> for details.
4. Engineering, can you please confirm the first part of #3? (all 3 are ignored, and nothing else is ignored? e.g., trans/rot?)