Answer

See P20.001(Duplicated below)

There is no one distinctly definitive element as they are both specialist subcontractors. I trust that you have a copy of the Principal Building Agreement available. Compare the following clauses:
20.0 Nominated 21.0 Selected
20.1.1 No consultation with contractor 21.1.1 Consultation with contractor
20.1.2 Appointment as per 20.1.1 21.1.2 Appointment as per 21.1.1
20.7 Termination decision by PA 21.7 Termination decision by contractor
20.9 N default - Contractor compensated 21.9 Employer default - contractor compensated
Open to all tenderers Limited to selected tenderers
Open tender - preferred by State Limited tender - avoided by State
Employer’s risk increased Contractor’s risk increased