Zones are the unit of administrative deployment.
The set of zones is also the set of locations across
which data can be replicated. Zones can be added to or
removed from a running system as new datacenters are
brought into service and old ones are turned off, respectively.
Zones are also the unit of physical isolation: there
may be one or more zones in a datacenter, for example,
if different applications’ data must be partitioned across
different sets of servers in the same datacenter.