This section of the license, does it mean that if I have a closed-source project, and I change the Panda3d code itself, I must release my modifications? Or is it simply governing aspects of how I must document my modifications, IF I release the source to my project that includes modified Panda3d source?
This is just addressing distribution of your changes. If you make changes and keep them to yourself, that’s fine.
This clause is one of the weird ones that we’d like to rewrite in the next revision of the license to make things a little clearer (and to remove the strange requirement to send your stuff to us on one particular email address). But the wheels of Disney Legal turn very slowly. . . .