List of allowed PATCH fields?

I saw someone reference in another answered question that somewhere there's a list of which fields can/cannot be updated via the PATCH method. I looked through the API documentation and can't find it. If this does exist can someone help me find it?

I kept trying to retrieve the RequestorUID and then update the responsibility of the ticket with that id yesterday, for our Change tickets, and wasn't able to. I'm wondering if this is because of something special with the RequestorUID field. I got it to work by using the uid of the Creator instead, but am still curious about why it didn't work.

Thank you!