You can create building elements from the faces of mass instances. Use mass instances when you want to model a building abstractly, or if you want to schedule gross volume, surface, and floor area.
To create building elements from mass instances, use Building Maker tools. Building elements created with Building Maker tools do not automatically update when the mass face changes. You can update the element to adjust to the current size and shape of the mass face.
Use generic models if you must create a unique, unusual shape but do not need to model the entire building abstractly. Walls, roofs, and curtain systems can be made from faces in a generic model family.
For more information about Revit import capabilities, see Importing Massing Studies from Other Applications and Suitability of Imported Geometry.