Clamping performance and surge rating
The 1.5SMC510A-M3/9AT: The clamping voltage at the rated peak pulse current of 2.15 A is 698 V — that is the voltage the protected circuitry sees during a surge event, and it must stay below the downstream component's absolute maximum rating.
Package, footprint, and board integration
The SMC footprint is larger than the SMA/SMB siblings, providing a wider copper pad area for thermal dissipation during repetitive surge conditions — the junction-to-ambient thermal resistance benefits from the larger tab.