You must set a server as a hub if that server is going to connect to multiple servers at the same time. Hubs will allow connections from all servers defined whereas leafs will not allow more than one server connection at a time.
/as server ChatNow set hub
Hubs require more bandwidth than leafs, and losing a hub causes more problems than losing a leaf, so the hubs should be the stablest servers on the network. Often hubs allow fewer clients on, to allow them to focus their resources on holding the network together.