Theme in Embedded Clauses

Halliday & Matthiessen (2014: 127):

These are clauses that function inside the structure of a nominal group, as defining relative clauses, e.g. who came to dinner, the dam broke, requiring travel permits in the man who came to dinner, the day the dam broke, all personnel requiring travel permits. The thematic structure of such clauses is the same as that of dependent clauses. However, because of their downranking, the fact that they do not function as constituents of a sentence [clause], their thematic contribution to the discourse is minimal, and for practical purposes can be ignored.