[Building Sakai] Sakai realms ???

[Phuc Bui]

Dear all,
I do not understand clearly about the Sakai realm and role.
The realm id, such as !user.template.maintain, /content/attachment/, is there any convention to create new realm name?
1. What happens if I create new realm like this !user.template.my_name
2. What is the difference between the id like !xxx.xxx and /xxx/xxx ?
3. When should we create new realm myself ?
About the role:
When I create new site, I have the realm like this /site/54f3be76-7159-44be-8bf0-61cb73781511, and now I can add new role and assign permission for that role.
1. When I create a new role and assign that role to a user, that user cannot see the site. Although earlier he had the role Student and joined the site ?
2. When I create a new course site, I have 3 default roles: Instructor, Teacher assistant and Student. If I want a new default role for course site, I add new role to the realm !site.template.course ?

By the way, would you please show me some articles that tell about Sakai realm and role ?

Thanks for your interest
Phuc Bui