Final answer:
Mezzanines must be fire-rated for enclosed spaces.
Step-by-step explanation:
In the context of building codes and regulations, the word that is missing from the question is fire-rated. A mezzanine is a partial floor that is typically found in buildings, and if it is enclosed, all portions of it must be fire-rated. This means that materials used in the construction should have a certain level of fire resistance to prevent the spread of fire. Fire-rated materials can include fire-resistant walls, doors, windows, and flooring systems.