What this Snubberless triac does on your board
The STMicroelectronics T435-600T is a 4 A RMS, 600 V off-state Snubberless Alternistor triac in a TO-220-3 through-hole package. It switches AC loads without requiring an external RC snubber network.
Snubberless means one less passive
The Snubberless designation means this triac is rated for high commutation dV/dt without an external RC snubber. The trade-off is the higher gate trigger current (35 mA). The 35 mA hold current also means the load must draw at least that much to stay latched on.