Halliday & Matthiessen (2004: 221):
To differentiate them in analysis, we can apply textual probes: in principle, being an element of the clause, a circumstance is subject to all the different textual statuses brought about by theme, theme predication and theme identification. … In contrast, a Qualifier cannot on its own be given textual status in the clause since it is a constituent of a nominal group, not of the clause; so it can only be thematic together with the rest of the nominal group it is part of.