Halliday & Matthiessen (1999: 518):
The third type of constituent of the clause, referred to above as “others” (that is, elements that are neither verbal nor nominal groups), evolved as the representation of a kind of “third party’ to the process. This may be some qualification of the process in terms of its manner of occurrence (an adverbial group, in the case of English); or it may be an entity that is involved in the process but only indirectly (in English, a prepositional phrase, consisting of preposition plus nominal group).