Usable is the amount of space on the inside you can actually use. What you are describing is called Gross or Rentable square footage (RSF). What yo must know is that RSF can be negotiated to reduce rent if overstated.
All commercial deals I do are negotiated point by point. Every little detail and agreement that is put in place with the landlord is what accompanies you the entire time you are in the space. So price per foot, utilities, landlord's work, tenant's work, escalations, tax caps and pass through, garbage, HVAC, sprinklers, water, heat, electric, fixtures, use limitations, sublease provisions... and the list goes on.
If you've never rented commercial space, you need to have a professional walk you through it.