Halliday & Matthiessen (1999: 83):
Any option (category, semantic type) is thus located in the network relative to other options, first within its own system, and secondly in terms of the location of that system relative to others. For example, 'participant' is one option in the system 'participant/process/circumstance'; and that system is a more delicate elaboration of 'element'. We noted above in reference to Painter's work that the resources of figures of being are deployed in construing taxonomic relations. There is also a generalised set of nominal categories for construing steps in the delicacy hierarchy — e.g. type, kind, class; and some taxonomic regions have specific categories of their own, for example: brand, model, make, issue; genus, species, family.