This is what stratal tension means… why we need a stratified model of grammar and discourse semantics...
'Stratal tension' refers to instances of grammatical metaphor. Grammatical metaphor was Halliday's principal motivation for stratifying the content plane into semantics and lexicogrammar. The intrusion of the word 'discourse' before 'semantics' is redundant and potentially misleading. Halliday & Matthiessen (1999: 237):
Of course, what we are recognising here as two distinct constructions, the semantic and the grammatical, never had or could have had any existence the one prior to the other; they are our analytic representation of the overall semioticising of experience — how experience is construed into meaning. If the congruent form had been the only form of construal, we would probably not have needed to think of semantics and grammar as two separate strata: they would be merely two facets of the content plane, interpreted on the one hand as function and on the other as form.