Today's pop quiz: replication

[Nicola Nye]

I've nearly incorporated all of the information you fine folks have
provided, at least as far as common usage scenarios go, into the docs.

One final gap remains in my knowledge...

I have the impression that a replica can be set up to have a subset of
all the users/mailboxes of a master. Have I misunderstood something or
is this the case? If so, where would such a thing be configured? 

To date everything looks as if each replica is set up to be a clone of
the master, so each change gets written multiple times, one to each sync
log (one sync log per channel, one channel per sync client, one sync
client per sync server). 

Am I right in assuming that a 'channel' is a virtual construct, a piece
of nomenclature; something that defines the linkage configuration
between master and replica?

   Nicola