Halliday & Matthiessen (1999: 72-3):
Here one of the questions has been what resources are available in the semantic system for construing new meanings — for ‘category development’, both in the ontogenetic time frame and in the logogenetic time frame. One central semantic motif that has emerged from these studies is that of elaboration: both in relational clauses (realising figures of being & having) and in complexes, especially nominal ones (the traditional notion of apposition, realising sequences of participants). Here elaboration is at work ‘distilling’ new meanings over extended passages. That is, the semantic system includes a theory of how meanings are construed; and this theory is itself a resource for construing new meanings.
Thus when a young child says cats also aren’t people, he is using a type of figure of being & having to construe a taxonomic relationship in the ideation base, so that the membership of ‘cats’ (construed as Carrier) in the class of ‘people’ (construed as Attribute) can be probed, in this case to ‘outclassify’ cats from the class of people. Figures of being & having of the intensive and ascriptive kind thus construe, among other things, the taxonomic relation among classes or types in the semantic system. The ideation base is thus a resource both for construing experience and for construing its own construal of experience. It has the potential for expanding itself precisely because it includes a theory of how meanings are construed.