Camera serial numbers. If you ever have your camera stolen, you can put the serial number into Stolen Camera Finder. It will show you the images uploaded to the Internet with your camera’s serial number so you can recover it.
iPhone metadata includes several fingerprints that can be used to differentiate between different devices, including a serial number, uptime in seconds, and shutter release count (depending on the model of iPhone you have).
(source: me examining as many different iPhone models and instances to build deduping heuristics for PhotoStructure)
https://www.stolencamerafinder.com