Open Stack

Hi everyone,
I want to understand the different possibilities to deploy Openstack Nova in a site with thousands of hosts. After reading the documentation and following the list for some time now I’m confused how to use the concepts: “region”, “availability zone”, “cell”, “host aggregate”… to have a good logical division.
 
In the list was discussed that a “cell” (1 rabbit, 1 db, …) supports between 500-1000 hosts, so I understand that for a big site we need several “cells”.  Cells can be nested in a tree fashion where each “child” can have several “parents”. Can I say that an availability zone is one of these trees of cells?
 
Because a big site has heterogeneous hardware I can use the concept “aggregate host” for logically group similar resources. Aggregate hosts live inside cells.
Are these assumptions correct?
 
And how to deploy openstack nova between different sites (or regions…)? I saw some discussions for swift but none for nova.
 
thanks,
Belmiro Moreira