Jack,

Nominated s/c - someone the Employer has told the contractor he must use. The client will have usually negotiated the terms and price and entered a PC sum into the tender Bill of Quantities, which you add attendences, profit etc to, again usually in a seperate Bill item. There are often certain obligations that the Employer remains responsible for such as programme, but this entirely depends on what is written in the contract.

Domestic or direct are probably the same thing - ie a sub contractor the main contractor employs without reference to the Employer with regard terms and price. There may be specific conditions for their emloyment such as Employer approval and the incorporation of specific terms required under the main contract. The main contractor takes full responsibility for the performance of a domestic sub contractor and adds profit etc to the rates of work that the s/c carries out in the same way as any other rate.