Halliday & Matthiessen (2014: 154-5):
An Adjunct is an element that has not got the potential of being Subject; that is, it cannot be elevated to the interpersonal status of modal responsibility. This means that arguments cannot be constructed around those elements that serve as Adjuncts; in experiential terms, they cannot be constructed around circumstances, but they can be constructed around participants, either actually, as Subject, or potentially, as Complement … .