175°C junction — the thermal headroom you actually use
That 175°C TJ max isn't a number you design to continuously — derate the 125 W dissipation at high ambient — but it gives you margin in a motor-drive or DC-DC converter where the case runs hot. The TO-220 package with a tab soldered to a copper plane or bolted to a heatsink is the standard way to pull that heat out; without a heatsink the 100 A rating is theoretical, not practical.
