The nearest that the present grammar comes to a generalised concept of a 'modifier + head' relationship is its recognition of the fact that other elements of a group typically depend on the presence of the 'pivotal element'. Thus when the grammar generates a "common nouns" as the head of a nominal group, other elements realising other types of meaning typically get brought into play as well. Thus it is preferable to characterise the nominal group as a unit for expressing the wide range of types of meaning associated with a 'thing', rather than in terms of an over-simple series of 'modifier + head' relationships.
So far I have been explaining the concept of 'class of unit' in terms of the concepts of the Cardiff Grammar. In very broad terms, the concept is the same in IFG — except that the account of 'class of unit' given above is more directly connected to the level of meaning here than it is in IFG. However, the criteria used in the present theory for setting up different classes of unit are completely different from Halliday's, and the concept of 'class of unit' is therefore also significantly different. Indeed, the result is the recognition of a different set of classes of unit for English, as we shall see in the rest of Section 10.2.
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[1] This is misleading. To be clear, Halliday's SFL Theory models the nominal group as both an experiential structure ("types of meaning associated with a 'thing' ") and a logical structure ("an over-simple series of 'modifier + head' relationships"), whereas Fawcett's Cardiff Grammar only models the nominal group as an experiential structure. SFL Theory therefore has the added explanatory advantage of accounting for nominal group structures where the Thing and Head are not conflated, as in measure expressions (Halliday & Matthiessen 2014: 391-2):
[2] This is seriously misleading, because it is the direct opposite of what is true. In SFL Theory, the class of a unit, such as a group, is distinguished by taking the view 'from above', the function it potentially serves at the higher rank. Halliday & Matthiessen (2014: 363):
A class is not a grouping of members of a given unit which are alike in their own structure. In other words, by reference to the rank scale, classes are derived “from above” (or “downwards”) and not “from below” (or “upwards”).