I would expect that as the rotor rotates in it's normal direction, then the bit of slot nearest the centre / hub should enter the caliper first, and the outer end of the slot should enter last. If you were to put a pointer of some sort into the slot, then as the rotor rotates, the pointer would move from the inner to the outer end of the slot.
Viewing the driver's side, with the slot at the top of the rotor, the slot should look like , and the passenger side should look like /.
This orientation allows for hot gasses and other crap to be thrown to the outer edge of the rotor rather than being all bunched up near the hub.