Usually all repeater within a region will subscribe the same talk groups. This allows to operate in these talk groups in a repeater transparent way. Therefore, it does not matter which repeater is being used within the region, the same talk group remains reachable. In the region Berlin & Brandenburg, this it the TG2621.
It therefore makes sense to enter all repeaters into one list that have the same talk groups subscribed. If the radio now would automatically select a reachable repeater, one could drive around in the region and would stay connected to these talk groups irrespective of the own position in the region. This feature exists in many radios and is called Roaming. Many of the slightly more expensive devices support this feature (e.g., AnyTone). The cheapest ones usually do not[5].
To use this feature, all channels with a certain talk group should be added to a list. The so-called Roaming Zone. This could actually be done automatically, but the programming software for these devices is usually not very user friendly.
If the signal strength of the currently selected repeater falls below a certain threshold (usually -105dBm), the radio will start to search the roaming zone for a repeater which is stronger than this threshold. This only happens if the radio is in standby. That is, if neither something is received nor transmitted.
If the radio finds a stronger repeater in the roaming zone, it automatically changes to that repeater. The new repeater does not necessarily needs to be the strongest in the zone. It only needs to be stronger than the threshold. If no stronger repeater is found, the radio remains on the currently selected one.
This roaming can also be set to “manual”. That is, the roaming search will only start if the signal strength is lower than the threshold and the PTT is pressed or the search is started from the menu.
[5] This is somewhat weird, as this feature is a pure firmware feature and does not need any additional hardware.