Halliday & Matthiessen (2014: 666):
Every lexicogrammatical system realises some semantic system. For example, we have seen that the grammatical system of MOOD [clause] realises the semantic system of SPEECH FUNCTION [proposition/proposal]. Here one lexicogrammatical domain, the clause, corresponds to one semantic domain, the proposition/proposal. In addition, there are semantic systems that are realised by grammatical systems operating in more than one place in the grammar. These semantic domains range over two or more grammatical domains, spanning two or more grammatical units, as in the case of MODALITY noted above.