"When all or parts of a building are required to be fireresistive or have fire resistance, the model building codes typically require that the fire protection be achieved by some sort of fire-resistant rated 'assembly.' What is an 'assembly?' An assembly is a combination of materials put together in a specific way that achieves the fire resistance rating required in the building code. For example, today's illustration shows only part of an assembly: gypsum wallboard applied to a steel stud. In order to achieve a minimum fire resistance rating, both sides of the steel stud would have to be covered with fire-rated gypsum wallboard. Fire-resistance ratings apply only to assemblies in their entirety."
Coffee Break Training - Fire Protection Series No. FP-2009-47
United States Fire Administration: http://www.usfa.fema.gov/