Halliday & Matthiessen (2014: 561):
In an elaborating nominal group complex, the secondary nominal group may be used to include an embedded clause as Qualifier:||| Near the San Diego Freeway interchange is the huge Shell Chemical Company plant, part of an industrial district [[[ that was established || before the plain became almost covered with tract housing]]] . |||
Note also that the secondary nominal group may be delayed for textual reasons, giving rise to a discontinuous complex:While each of these elements is absolutely essential, || one must come first – people.
As with elaborating complexes in general, this is spoken on two tone groups, with tone concord.