The 1.5SMC13CA TR13 PBFREE: That 18.2 V clamp means a 12 V rail transient is held well below the typical 20 V abs-max of downstream regulators, so one TVS per rail covers the surge without a secondary clamp. This is a standard TVS selection: the standoff must sit above the nominal rail by enough margin to avoid leakage at the high end of the rail tolerance.
The SMC footprint is common for 1500 W TVS diodes — the copper pad area under the cathode tab sets the thermal resistance, so the board layout must match the recommended pad geometry to achieve the rated peak pulse power. The bidirectional construction means a single device protects both polarities — no need to orient the cathode on the PCB.