In Vasari, the basic building block is called a mass. A mass is a wrapper for a form, or series of forms, which provides the ability to query certain appropriate properties. For example, once you create a mass, and add mass floors, you can query items such as floor area, surface area, and volume.
Masses can be at the scale of a building, the scale of a wing of a building, or even at the scale of a unit. They can be used to calculate gross floor areas, surface areas, and volumes, analyze solar radiation, and analyze energy use.
Secondly, if you want create a mass that can be modified in context, click the Create Mass button. You can then start modeling immediately within the context. To finish, simply double click in a blank area of the view, or click the Finish Mass button.