Site roaming is the ability for a radio to work at two or more different site repeaters without the radio user changing channels or pressing any buttons.
Repeaters operate on radio frequencies issued by OFCOM. It is very expensive to have the same frequencies issued at two or more sites so frequencies differ from one repeater to another. This means a radio needs the capability of operating on two or more frequencies. Traditionally, in a non-roaming system this means the user changing channel when moving between sites.
In a Roaming radio system each repeater transmits a unique Beacon signal every few seconds. The roaming radio hunts for this beacon. Once a beacon is found the radio configures itself to work with that repeater. If the radio moves out of range of the repeater the beacon signal is lost and the radio starts hunting for the beacons of other repeaters which are stored in the radio memory.