In value studies functions exist in two categories: basic and secondary. (Function models further contain subsets such as higher and lower order functions unique to that technique).
A basic function is the primary purpose(s) for which the item or service was designed when it is operating in its normally prescribed manner. This function must be accomplished to meet the purpose of the product, structure, or service. A product or service may have more than one basic function.
Secondary functions are ones that support the basic function (and, hence, are sometimes referred to as “support functions”). They result from a specific design approach to achieve the basic function. If the design changes, the need for existing secondary functions may be modified or even eliminated. To enhance the analytical an evaluation process some
practitioners break secondary functions into a sub-classification of “required” (by the current design), “aesthetic,” and “unwanted” such as the “emits heat” function of an overhead projector.