A sub-client can see all the clients on the system in the client drop down list
If a client creates it's own client (a sub-client) the sub-client, when adding a DNS zone, can see all the clients on the system in the client drop down list.
If a client creates it's own client (a sub-client) the sub-client, when adding a DNS zone, can see all the clients on the system in the client drop down list.