That would be your trim pin I think. If you have a power trim eng then put it next to the transom. If it is not a power trim eng you will need to try different holes and see where your boat handles best.
The reverse lock is the hook mechanism that catches onto the trim pin then?
The motor doesn't have power trim.
Yes, the reverse lock engages the trim pin. As mentioned, set the trim as needed for best boat performance. Usually you need to set the trim so the anti-ventilation plate is parallel with the boat bottom.