Halliday & Matthiessen (1999: 96, 97):
A phenomenon of experience is construed as a category in the ideation base by being given a location in the three semantic networks described in the preceding section. This taxonomic, meronymic and eco-functional location is the category's value (valeur) in relation to the other categories within the ideation base. By being assigned a value internal to the ideational system, a semantic category is also being related to categories that lie beyond semantics itself. On the one hand it is being related to categories within systems that lie outside language but which the semantic system interfaces with; on the other hand it is being related to the grammatical categories in terms of which it is realised. Figure 2-18 shows these three strata! angles on the process of construal.
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Importantly, these are categories of perceptual semiotic systems. SFL Theory makes the assumption of immanence: all meaning is within semiotic systems.