According SIA ordnance 416, the main usable floor area (Hauptnutzfläche or HNF) is that part of the usable or occupiable floor space (Nutzflächen or NF) that specifically serves the purpose of a bulding's designated use.

It does not include load-bearing walls and external walls (construction areas), secondary areas (e.g. cellars). Somewhat more simply defined: It is the sum of all accessible and occupied living areas including the non-load-bearing interior walls. 

Here an illustration of the standard SIA 416 (Source: Kassensturz Schweizer Fernsehen)