Wi-Fi devices also have a MAC address (6-octet, typically displayed as hex). This is unique but since it's not available on cellular devices and not convenient to use, it's generally not recommended as a unique identifier. Additionally, devices connecting by Ethernet will have a different MAC address on the Ethernet LAN. There is also a BLE MAC.
From user firmware, use
WiFi.macAddress() to get the Wi-Fi MAC address.
The MAC address is available by the Particle cloud API get device information in the