Halliday & Matthiessen (2014: 388-9):
… seeing [the nominal group] as a logical structure … means seeing how it represents the generalised logical-semantic relations that are encoded in natural language. … for the purposes of the nominal group we need to take account of just one such relationship, that of subcategorisation: ‘a is subset of x’. This has usually been referred to in the grammar of the nominal group as modification …