A volume is a storage area to synchronize. It can be located in a company connected to Internet or in a data center.
The hub transfers the files between the connected volumes. It is installed in a datacenter on a high availability machine to provide a viable, optimal and continuous service.
The tracker is a server dedicated to the synchronization of remote volumes. Its dashboard (below) show a global view of the synchronization, with different options, the connected volumes, hubs and links.