Halliday & Matthiessen (1999: 94, 58):
Taxonomic elaboration and meronymic extension form complementary networks of relations that together make up the paradigmatic organisation of the ideation base. They define, as we have noted, options in delicacy of categorisation and in delicacy of focus. The third type of network involved in the organisation of the ideation base which we identified above serves to relate paradigmatic organisation and syntagmatic organisation. Specifically, it relates syntagmatic functions or roles associated with paradigmatic types to the paradigmatic types that can serve in these functions. For example, the syntagmatic function 'Senser' is associated with the paradigmatic type 'sensing', and it is related to the paradigmatic type 'conscious being', since only participants of this subtype can serve as Sensers.
Figure 2-6 above shows this relation, together with a set of other such relations. We shall refer to such network relations as eco-functional selection in order to indicate that they specify the syntagmatic environment of semantic types by showing them as selections for syntagmatic functions. Such selections have been referred to as "pre-selections", but in order to avoid any connotations of temporal sequence, we prefer the term "selection" for such relations in the ideation base.