For proposals for the commitment of expenditure, the following draft acts constituting obligations on the part of the institution shall constitute supporting documents: (a) for expenditure incurred by virtue of contracts: the draft agreement, contract, order form or similar document, accompanied, in the case of contracts, by the tenders received, the reasons for private treaty and, where appropriate, the opinion of the Advisory Committee on Procurement and Contracts (hereinafter referred to as'the CCAM').