about NEMA 6-15 plug/receptacle for 240 V
That's a ground hole and goes to the ground bar

The NEMA 6 is a "hot-hot-ground" that supplies 240V exclusively.  So, the U-shaped ground-hole on the receptacle is connected to the yoke and in turn, the green ground screw on the receptacle, which needs to be wired to the ground bar on the subpanel.