The NF X50-151 standard is the French AFNOR standard that defines the framework for the functional analysis of a product or a system. It governs the functional expression of need — carried out in practice by the bête à cornes and the pieuvre, both from the APTE method — and requires functions to be characterised by figures, through the criterion / level / flexibility triplet.
Getting the standard
The official standard is not free: it is sold on the AFNOR website. For a first educational look, see the article Analyse fonctionnelle (conception) on Wikipedia.