A Revit ruleset defining how an area calculation - Gross, Rentable, Usable - is measured across a project. An Area Scheme defines the measurement rules behind an Area Plan - whether boundaries are drawn to the wall centerline, the interior face, or a leasing convention like BOMA - so a project can maintain several distinct area calculations (Gross Building, Rentable, Usable) side by side without them interfering with each other.