Final answer:
The unfinished space between the ceiling and roof assemblies is known as the attic or loft, and in commercial buildings, it could also be referred to as a plenum.
Step-by-step explanation:
The unfinished space between the ceiling assembly and the roof assembly in a building is often referred to as the attic or loft. This area can be used for storage, or it may house systems such as HVAC, plumbing, or electrical systems.
In homes, it's also commonly insulated to help control the temperature within the living spaces below. In commercial buildings, this space might be referred to as a plenum, especially when it is used for passing heating, cooling, and ventilation systems.