Halliday &
Matthiessen (2014: 424):
The prepositional phrase serves as Adjunct in the modal structure of the clause. Like the adverbial group, it can serve as circumstantial Adjunct or, less commonly, as interpersonal Adjunct; and like the conjunction group, it can serve as conjunctive Adjunct. In addition, it can be rank-shifted to serve as Postmodifier in a nominal group or adverbial group.