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A "geofence" is a virtual boundary associated with (usually surrounding) a geographic area. Devices crossing a geofence can trigger a response or report the event.
For example, a geofence surrounding a school could report devices entering or leaving, such as student phones or school-provided pads or laptops.
Advertising can be targeted by a geofence - surrounding a bridal store, divorce lawyer, or medical center, for example.
You may trigger dozens a day.