Hi Ben, I don't quite understand your question.
Resource calendar shows resource availability to aid you in your schedule planning. Let's say you need someone to work on task A on day 1 to day 5 and you notice that Person A is available, then you will update the resource calendar indicating that Person A will not be available day 1 to day 5 since he will be working on task A. This will also show whether Person A is available for other projects or not (in case some other project is eyeing on the same person).
I hope the above answer your question. If not, please post your question again with an example.
Are resource calendars updated when team members are assigned project work on the project schedule? In other words, is their availability for project work updated based on project work that has already been assigned to them?