[1] HVAC airflow in spaces generally can be classified by two different types: mixing (or dilution) and displacement.
The fan coil or handling unit takes in return air from the ceiling void and mix this with fresh air and cool, or heat it, as required to achieve the room design conditions.
[2] Displacement ventilation systems supply air directly to the occupied zone.
The air is supplied at low velocities to cause minimal induction and mixing.
Displacement room airflow presents an opportunity to improve both the thermal comfort and indoor air quality (IAQ) of the occupied space.