Halliday & Matthiessen (2004: 592):
… there are semantic domains that range over more than a single grammatical unit. Thus the semantic domain of modality is construed in more than one place in the grammar; for example, it is construed by clauses such as I suppose and it is possible, by verbal groups with finite modal operators such as may and by adverbial groups and modal adverbs such as perhaps. These modal patterns within different grammatical units are not interchangeable/synonyms; they have distinct values within the overall semantic system of modality. … This means that the semantic system of modality is more extensive than the modal features of any one given grammatical unit would suggest; it is realised not by a single grammatical unit but by a range of units: semantic unit a ø grammatical units m, n and o.