Halliday & Matthiessen (2014: 158):
The two types of Adjuncts are also similar both in their own composition (as adverbial groups and prepositional phrases) and in how they may be differentiated from circumstantial Adjuncts. Whereas circumstantial Adjuncts fall most naturally at the end of the clause, where they carry the unmarked tonic (intonational) prominence, modal and conjunctive Adjuncts occur finally only as Afterthought and can never carry the only tonic prominence in the clause. … And while they all can occur thematically, only the circumstantial Adjuncts can normally occur as predicated Theme …