The STP03D200: The Darlington pair gives a minimum DC current gain (hFE) of 200 at 30 mA, 10 V, so the base drive requirement stays low even when switching the full collector current. Maximum power dissipation is 40 W, and the Vce saturation is specified at 2 V maximum with a 500 µA base current driving 50 mA collector current — a useful figure for estimating conduction loss in a switching or linear application.
The tab is the collector — the heatsink interface needs electrical isolation unless the entire assembly shares the collector potential. Supplied in Tube packaging, which is the standard for through-hole power devices and works well for manual insertion or low-volume assembly.