a quantitative expression of the behaviour of a construction product, for an action to which it is subject or which it generates under the intended use conditions. Levels of product performance can relate to the product as whole or to individual characteristics or combinations of characteristics. They can be used to define a construction product for a specific intended use, to set a minimum performance below which a product cannot in any circumstances be considered fit for that use (threshold levels) or as a basis for the establishment of classes of product performance.