Bugfender automatically logs a device property called $android_id. This identifier is described in the Android SDK documentation:
Some quick notes about this identifier:
This ID is supposed to be constant for the same user on the same device. Different users sharing a device might have different IDs.
Factory resetting a device regenerates the Android ID.
There is no 100% guarantee that this identifier is unique, as it a random ID. Actually, some custom Android ROMs have a fixed ID for all devices, or it is sometimes null.