As HRV systems are not 100% efficient, supply air from them will be slightly cooler than the general air in the house and if supplied at a high level in the room, will drop slowly and mix with existing air. For this reason, supply terminals can be at high level, either wall mounted or ceiling mounted.
Wall mounted terminals allow a slight pressure to be generated such that the air will project across the ceiling in a coanda effect and drop into the room before being drawn slowly in the direction of the extract point usually in a nearby bathroom or kitchen.
Ceiling mounted terminals need to operate slightly different, as they project air at lower velocities in all directions at a point in the room which is deep and as far as possible from the nearest extract point. The idea is that if you were to sit under one of these valves you would not feel any uncomfortable draught.
Extract terminals work in a different way, they create a slight vacuum in the area around the terminal. This causes a movement of air into this space to fill this vacuum and this air generally moves horizontally along the ceiling. In the maritime climate of the UK where humidity is relatively high in winter, these extract points should be close to the areas of moisture generation, such as over showers etc.