Interstitial vs. Plenum space

Interstitial space

An interstitial space is an intermediate space located between regular-use floors, commonly located in hospitals and laboratory-type buildings to allow space for the mechanical systems of the building. An interstitial space is useful when the mechanical system of the building is highly sophisticated and changing the space on the primary floors is a distinct possibility. The heights of these spaces are generally six to eight feet and allow easy access for repair or alteration. The entire floor plan of these buildings can be more open because there are fewer fixed vertical penetrations through the floor and walls.

Hypothetical interstitial space design for a medical facility.

source: https://en.wikipedia.org/wiki/Interstitial_space_(architecture)

Plenum space

 A plenum space is a part of a building that can facilitate air circulation for heating and air conditioning systems, by providing pathways for either heated/conditioned or return airflows, usually at greater than atmospheric pressure. Space between the structural ceiling and the dropped ceiling or under a raised floor is typically considered plenum. The cavity/plenum space is typically used to house the communication cables for the building's computer and telephone network.

A commercial building with a plenum airspace.

source: https://en.wikipedia.org/wiki/Plenum_space