Clarify scopes

[mwinters0]

The scopes docs left me with many questions.

1: What is the relationship between scopes and variables?

• While there seems to be a correlation between e.g. the URL variables and the url scope, it's never stated that those variables must be used in that scope.
• There is a domain scope, but there are no "Domain variables". So what variables can be used here? Only global?
• If I know what global variable I want to evaluate (for example, from.domain) then how do I know what scope to use? from.domain seem to fit all of these:
  • I am evaluating a domain
  • From the CC header
  • Which contains email addresses

2: Which parts of the message are processed by each scope?

• Some scopes like header and body seem pretty straightforward.
• Others like email and domain could apply to several sections. (For example, headers and envelope.)
• Some like url are completely unclear. What's the difference between this and e.g. header?