A Hub acts purely as a point for handling the orchestration of it's local GeoNetwork. It provides infrastructure to the network. It acts as a point for making multi-cast requests to all other nodes on the network; a point for resolving URIs to addresses or other networking interfaces; a gateway for other outside spring networks.
When a node wishes to join a GeoNetwork, it will perform a registration with one of the network's primary nodes which act with the role of a Hub. Once registered the Hubs will be able to perform role dependent actions on the node.