A Relay server can specify one parent server when it is started from the command line. Connecting to a parent enters the server into a server hierarchy. Adding servers to a hierarchy keeps the clients/server ratio to an efficient level as the number of clients increases.
If a parent server is specified, the server will establish a connection with the parent and get a current version of the shared state. From then on, the server treats its parent just like any other client; the server communicates with its parent by receiving and sending updates. The exception to this is that servers and their parents do not echo each others updates; as this would cause endless propagation of an update between a server and its parent.