What are the difference between direct subcontractor and nominated subcontractor?

a nominated subcontractor, with various provisions for obtaining indemnities by the subcontractor (or Employer) but under the main contractor responsibility and coordination, or
** another contractor, which FIDIC provides the incumbent Main Contractor is required to give site access, etc, etc to but coordinated by the Employer

My own understanding, very simply put is that a nominated subcontractor is actually a "normal" subcontract between Main Contractor and Subcontractor, BUT that the nominated subcontractor is selected by the Employer and price agreed with the Employer directly. That would mean that the Main Contractor has the accountability for the subcontractor, notwithstanding any indemnities.

The Consultant has stated today that in their opinion:
** the nominated subcontractor is selected by the Employer and signs a contract direct with the Employer
** the Employer will secure assurances, bonds / performance sureties and warranties direct with the subcontractor, and
** that the Main Contractor DOES NOT sign a subcontract with the subcontractor
** that the main Contractor will not be held liable for certain specified warranties

My understanding is that the Main Contractor still should sign a subcontract with the nominated subcontractor and assumes the responsibilities of a Main Contractor over the Subcontractor - though the Employer states that the Main Contractor has no liability specified warranty issues and quality (only).

I would believe the Main Contractor still has a number of responsibilities in respect of the subcontractor and their general performance.

The Consultant assures they have checked FIDIC and it is normal for the Employer to sign a subcontract direct with the Nominated Subcontractor.

I would believe in that case it is not a "Nominated Subcontract", but instead a direct supplementary contract between the Employer and their Contractor (secondary contractor)

Can you kindly advise quite simply, the contractual position of a nominated subcontract under FIDIC ...... ie does the Employer sign the contract / subcontract with the Nominated Subcontractor, or is it a subcontract between Main Contractor and Nominated Subcontractor with the normal (in general terms) obligations by the Main Contractor for the Subcontractor, to the Employer.