Every cellular device has an ICCID. For devices with a SMD Particle SIM, this is constant. For devices with an external SIM card (Electron), the ICCID depends on the SIM card. For devices that have a selectable SIM (Boron), the ICCID will depend on whether the internal or external SIM is selected, and which SIM is present if external is selected.
The ICCID 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.
Since the ICCID has this variability, and is not present on Wi-Fi devices, it's not recommended as a unique identifier.
The ICCID is available by the Particle cloud API get device information in the
iccid field. This is the last ICCID used to connect to the Particle cloud from this Device ID.