Round nosing (bullnose) has the same height as the wood floor and it makes the transition from the floor to a step down or a staircase.
Quarter round is a decorative moulding used for ornamentation or finishing.
The standard dimension of the wood vent is 4 x 10 with a ¾ thickness which fits the regular wood flooring specifications. It can be flush mount (same height as the floor) or top mount. Custom sizes are available.
Square nosing is the same as the round nosing, the only advantage is the contemporary look.
Reducer is to be placed between two surfaces of different heights.