Halliday & Matthiessen (2014: 559):
Group and phrase complexes are formed out of series of nexuses just as clause complexes are formed out of series of clause nexuses. Groups and phrases form nexuses in the same way that clauses do, by a combination of parataxis or hypotaxis with some type of logicosemantic relation; the different possibilities are set out in Table 8-1. Only elements having the same function can be linked in this way. Typically this will mean members of the same class: verbal group with verbal group, nominal group with nominal group, and so on. But it also includes other combinations, especially: adverbial group with prepositional phrase, since these share many of the same circumstantial functions in the clause; and nominal group with prepositional phrase, as Attribute (e.g. plain or with cream).