Halliday & Matthiessen (2014: 534):
The structure of a nominal group with an embedded projection is shown in Figure 7-22, part [i].
Such instances are still projections; but here the projecting element is the noun that is functioning as Thing, in this case assertion. … such instances of projection are all metaphorical: a projecting sequence is realised congruently as a clause nexus of projection – part [ii] of Figure 7-22, but it may alternatively be realised metaphorically as a nominal group – part [i] of Figure 7-22. When we align them as in Figure 7-22, we see how the nominal group construction with an embedded projection clause is agnate with a clause nexus of projection: the nominal group is a metaphorical, nominalised version of the clause nexus; and the noun assertion serving as Head/Thing is in fact a nominalised variant of the verb assert serving as Process in the agnate clause. The congruent Sayer may be left out in the nominal group; or it may be represented either as the Deictic (their assertion that ...) or as a Qualifier (the assertion by the government that ...). Part of the rhetorical power of the metaphorical group is the potential for leaving the Sayer unspecified.