IMEI
Also called: international mobile equipment identity
The IMEI (International Mobile Equipment Identity) is a 15-digit number that uniquely identifies a mobile device on cellular networks. It is tied to the hardware rather than the SIM, so it stays with the handset when the SIM changes, and it is one of the strongest signals in a device fingerprint.
Every device with a cellular radio ships with an IMEI burned into it. The number encodes the manufacturer and model in its first eight digits — the Type Allocation Code — followed by a serial portion and a check digit. Because it belongs to the hardware and not the subscriber, swapping SIM cards does not change it.
Why it carries weight
The IMEI is difficult to fake convincingly. Its first digits must correspond to a real manufacturer allocation, its check digit must validate under the Luhn algorithm, and the model it implies has to agree with everything else the device reports. An IMEI claiming a Pixel 7 on a device whose build.prop says Xiaomi is an immediate contradiction.
This is also why emulators struggle. An emulator has no cellular radio, so it has no genuine IMEI — it reports a default, a zero, or a generated value that does not correspond to any real allocation.
In a fleet of devices
When you operate many devices, each needs its own IMEI, and each IMEI needs to stay attached to the same device over time. Two devices sharing one IMEI are trivially linked. A device whose IMEI changes between sessions looks like a different phone every time it connects.