SELinux question: Label, type, roles etc.

Hi all,

I am trying to learn some SELinux stuff and was going through a lot of
material from NSA SELinux website and I have few questions after
reading the documentation:

1. What is the relation between the security context and label?
2. How is the label defined in SELinux (format etc.)?
3. What is the validation mechanism used for the security server for
granting the permissions?
4. How is SID related to the PSID (persistant SID)?
5. What is a *subject* in regard to SELinux policies.... I know a
process is considered a subject. But can a user also be considered a
subject? In short what things can be considered subject?

I would appreciate any help/pointers in this regard.

Thanks

Ashish