This integration creates a composite device_tracker entity from one or more other entities. It will update whenever one of the watched entities updates, taking the "last seen" (and possibly GPS and other) data from the changing entity. The result can be a more accurate and up-to-date device tracker if the "input" entities update irregularly.
It will also create a sensor entity that indicates the speed of the device.
Currently any entity that has "GPS" attributes (gps_accuracy or acc, and either latitude & longitude or lat & lon), or any device_tracker entity with a source_type attribute of bluetooth, bluetooth_le, gps or router, or any binary_sensor entity, can be used as an input entity.