Halliday & Matthiessen (1999: 565-6):
Any given model of experience exists at different orders of abstraction. It is a configuration of higher-level meanings within the context of culture; at the same time, it is also construed semantical!y in the ideation base. The relationship between these two orders of abstraction, contextual and semantic, is a stratal one; hence a model is a cultural construct that is construed in language (together with other, language-dependent semiotic systems such as expository drawings and diagrams).