Which makes a rat. Go look it up. What are you buying in a condo? The space. period and those are measured above the ground. You need to buy at least one appraisal textbook in your life. FNMA's Selling Guide is NOT a textbook. Whether those rights are owned outright or shared with the other Condo owners, you are basically getting the right to use a space above the surface.
Most condominium ownership is structured around the concept of air space. This means that the building, walls, and common areas are owned communally by all owners through the homeowners association (HOA). Thus, the individual condominium owner only controls the air space created by the walls of their unit.