Complex sentence: Space between the structural ceiling and the dropped ceiling or under a raised floor is typically considered plenum; however, some drop ceiling designs create a tight seal that does not allow for airflow and therefore may not be considered a plenum air-handling space.
Split into simpler sentences: Space between the structural ceiling and the dropped ceiling or under a raised floor is typically considered plenum. Some drop ceiling designs create a tight seal that does not allow for airflow and therefore may not be considered a plenum air-handling space.