Open Stack

Trying to get a good idea of how they work.  As I get it, host 
aggregates are collections of (not necessarily mutually exclusive) hosts 
that share a metadata value.  And they can be part of availability zones 
which can be (say) specific to networking hardware, or CPU, or 
something.  What I'm trying to accomplish is something where generic VMs 
being fired use the default zone, "nova", but *only* ones where I 
specify a different zone -- say, superfastcpu -- will make use of that 
zone.  Do I have it right?  Or will VMs that I fire up where I don't 
specify a zone eventually make use of the superfastcpu zone?

Thanks!

-Ken