This article deals with the analysis of the part positioning in models of mechanical assemblies and its systematic recording. This analysis is essential in the process of obtaining the functional dimensioning of each component that meets the geometric requirements set by the specifications. The method is derived from knowledge of the nominal models of each component and the whole assembly. The first phase of functional dimensioning is based on the analysis of the positioning of each part in any possible operating state. The part positioning depends on the identification and definition of the contact surfaces and their order of preponderance. This method makes it possible to obtain the dimensioning of the contact surfaces by means of position specifications. In addition, by precisely defining the part positioning, the ISO dimensioning can be carried out in such a way that the consistency of the dimensioning with the requirements can be easily verified. Every functional requirement will generate a tolerance stackup of influencing dimensions that must be recorded. Considering that an assembly or device may have several operating states, parts may have several operating positions which shall be analyzed and provide different dimension tolerance stackups and functional dimensions. For the above reasons, the method must use the language common to all these environments, which is none other than the ISO-GPS language.