Halliday & Matthiessen (2014: 34):
The ideational, interpersonal and textual meanings at risk can be stated in terms of systems at the semantic stratum in the first instance. However, since semantics stands in a natural relation to lexicogrammar, the two being the content plane systems of language, meanings at risk can also be stated, by another stratal step, in terms of systems at the lexicogrammatical stratum as wordings at risk. For example, when we consider the correlations between tenor values and terms in interpersonal systems, we should really focus on interpersonal semantic systems such as SPEECH FUNCTION in the first instance rather than on lexicogrammatical ones such as MOOD.