For a location that has sub-zones within it (e.g. an AWS region has availability zones that can be
mapped as sub-locations), this extension interface allows those to be accessed and used.
For some well-known clouds, the availability zones are automatically set, although for others they may
have to be configured explicitly. The "multi:(locs,...)" location descriptor (cf
this to be down at runtime.
Note that only entities which are explicitly aware of the
will use availability zone information. For example
Implementers are strongly encouraged to extend
which has useful behaviour, rather than attempt to implement this interface directly.