Halliday & Matthiessen (1999: 27):
The concept of reactance is particularly significant for our purposes where it involves a relationship between an ideational category and features of other metafunctions, interpersonal or textual. For instance, the interpersonal grammar provides for participants, within the ideational dimension of the clause, to function as Subjects; but this potential is not in general open to circumstances, and this is a principal reason for distinguishing these two classes within the ideational metafunction.