Useable area is the actual sqft of premises occupied by a tenant, (including pillars, perimeter HVAC systems etc.). Rentable area includes the tenant’s useable area plus the tenants proportionate share of common area in the building that it, it’s visitors & service providers use. (Ie. common hallways, electrical rooms, bathrooms, etc.).
The rentable to usable calculations determines the floor and / or the building efficiency. Rentable and Useable calculations are generally determined by landlord’s architect and consultants.
Keep in mind that 2,000 sqft of useable space on different floors in the same building can have different rentable areas, [and thus differ in cost], depending on the efficiency of the common areas on each particular floor.