Each domain can consist of one or more sites. Sites can be used to distinguish between geographically distinct areas that are serviced by the helpdesk, or maybe just different departments within the same building. The crucial point to make is that clients and support agents (and every other type of user in the system - see Users - The Life Blood) are allocated their access on a site by site basis. As such, a client of one site does not have to be a client of all sites. This means that for a large helpdesk that is internally partitioned with different teams supporting different sites can be implemented simply with one domain split into a number of sites.
This allows for the system to be used for anything from a single support agent supporting a single client up to a city wide educational ICT helpdesk supporting a large number of schools and consisting of complex heirarchies of staff. That's the plan anyway!