Every cellular device has an IMEI, which is unique for device and stored in the modem chip (not the SIM). This will likely only come into play when using the device in a country that requires mobile device registration. Often, mobile device registration is tied to the IMEI because it does not depend on which SIM card is used.
The IMEI can be queried by using
Cellular.command() but there isn't a convenient method in the
Cellular class so using it is not as convenient.