Halliday & Matthiessen (1999: 9-10):
[A] sequence of classes is called a syntagm to distinguish it from the (function) structure. It specifies constraints on the units serving within the clause — the units on the rank below the clause on the grammatical rank scale. The rank scale determines the overall constituency potential in the grammar: in English, clauses consist of groups (/ phrases), groups consist of words, and words consist of morphemes. The units below the clause on the rank scale are all groups (nominal, verbal adverbial, etc.) or phrases (prepositional phrases), or else clauses that are shifted downwards on the rank scale to serve as if they were groups or phrases. Such down-ranking is known as rankshift. This has the powerful effect of expanding the resources of grammar by allowing the meaning potential of a higher-ranking unit to enrich that of a unit of lower rank.