 A general version of a ‘part-of’ relation between classifications is needed in order to model the way parts of a system are assembled together. Consider the case of the monitoring systems. That each one of them has a screen as one of its parts means that there is a function that assigns to each instance of the classification MonitSit an instance of Screens. On the other hand, all the ways in which a screen can be classified (the types of Screens) intuitively correspond to ways in which the whole screening system could be classified: if a screen is part of a monitoring system and the screen is blinking, say, then the whole monitoring situation is intuitively one of the type ‘its screen is blinking’. Accordingly, a generalised ‘part-of’ relation between any two arbitrary classifications \(\mathbf{A}, \mathbf{C}\) can be modelled via two functions 