In the project environment, any joined and overlapping mass faces split into 2 faces: interior and exterior. This behavior allows you to create interior or exterior hosts by face.
In the following illustration, 2 masses are joined, and the overlapping faces have 2 different hosts. The exterior face has a curtain system by face. The interior face has a wall by face with a door.
When you join masses, the area of the inner wall shared by the masses is deducted from the gross surface area for each mass. If you create mass floors, this inner wall area is also deducted from the exterior surface area for each mass floor.
The perimeter of a mass floor, however, includes the entire perimeter, even when it overlaps the perimeter of a mass floor in an adjacent joined mass.