Halliday & Matthiessen (1999: 82-3):
From the typological point of view, construing experience in terms of categories means locating them somewhere in this network of relations. When we examine this network more closely, we find that there are actually three types of network involved:
(1) taxonomic in the strict sense (i.e. based on hyponymy, 'a is a kind of x/ x subsumes a, b, c'),(2) taxonomic in the extended sense (i.e. based on meronymy, 'd is a part of y/ y has parts d, e, f'), and(3) eco-functional (i.e. based on selection, 'g has function m in environment z/ environment z comprises functions m, n, p, and function m may be filled by g, h, j').
Of these three, the first provides the global organising principle of elaboration in delicacy; while the third relates paradigmatic organisation to organisation on the syntagmatic axis.