The contract document is anything but a sacred dossier which is a taboo to discuss. For professionals like myself, i often trigger or activate clause(s) of contract for clarification and validation. Regardless in which ever party you are in for a contract binding, the game remain the same.

2.0 So what takes precedence of a contract document? It depends on the contract form you are using as Condition of Contract (CoC). Let us look at two generic forms; PWD 203 and PWD 203A. The main difference between both is the prevalence of either standard drawings and specifications (203) or Bill of Quantities (203A). In such arrangement, at the preamble of the CoC plays the important role in validating the importance of each document based on hierarchy of the Contract Document.

3.0 Based on Clause 1.1(b) for PWD 203, Contract drawings and specification does not take precedence directly in any order or hierarchy and is render as complementary to one another. The same goes with Clause 1.1(b) for PWD 203A.

4.0 Subsequently, the explanation in Clause 8: Contract Documents does not emphasize on hierarchy of either the Drawings and Specifications or Bill of Quantity. The only additional requirements for PWD 203 are the specifications, unpriced provisional bill of quantities and schedule of rates, while PWD 203A emphasized on priced Bill of Quantity.

5.0 The precedence for these forms only prevail in Clause 25: Valuation of Variation and 26: Summary of Tender (for 203) or Bill of Quantities (for 203A).

6.0 Both tender forms compliment their Clause 8 through validated actions by the use of contract drawings and/or the specifications in explaining and amplifying the true meaning or intent of the contract in order to complete work accordingly as long as it does not impose any obligation beyond those imposed by CoC. In this case, the use of drawings and/or specifications is/are justified and the calculation for variation of works shall be mediated through Clause 25 and 26 respectively.

7.1 Now, in certain cases where CoC was revised to fit certain project delivery mechanism, the context or even the pretext of contract is spelled out clearly in the preambles in Clause 1 in regards to precedence of annexed contract documents. For cost controlling, the customized contract form will reflect closely to the PWD 203A.

7..2 Can these be challenged? Yes, all of these can be challenged if it is proven that the Contract Documents are insufficient, where;

a. This is clearly highlight under the term of mutually explanatory of each other unless sole or exclusivity is spelled out in this sub-clause. 
b. Second challenge or exploit shall be the term or definition "proper execution of Works until its completion according to the true intent and meaning of this Contract taken together whether the true intent and meaning may or may not be particularly shown or described PROVIDED THAT it can be reasonably inferred therefrom.
c. Third challenge or exploit comes from discrepancy in or divergence between any two or more of them PROVIDED ALWAYS that such discrepancy or divergence shall not vitiate this contract.
d1. Fourth challenge is out of the clause for contract sufficiency when taking on specifications. This is closely related to contract clause 21: Compliance with the Law. In this case, "contractor should comply in all respects (including the giving of all notices) with any law, regulation or by-laws, or any order or directive issued by any public authority or public service company (hereinafter referred as "Statutory Requirements"), relating to the Works or, in the case of public.." Shall there be any change or amendment in any written law, regulations and by-laws which necessitates any variation to the works, a written notice should be made. 
d2.In addition to this clause would be Clause 10: Obligation of the Contractor; where the highlight [10.1(f)] to change should keep the [10.1(e)] Government/Client indemnified by [10.1(b)] performing the works in a proper manner and in accordance with good management practices, and exercising professional judgment and practice, requisite skill, care and diligence [10.1(d)]. Without prejudice, the party have also discharged their obligation appropriately [10.1(d)].

8.0 Therefore, you can definitely challenge the precedence of documents within the Contract Documents through the CoC which takes higher precedence than any other annexed supporting document. Regardless of what the preambles referred to or rendered, CoC shall be read in the light or in conjunction with other annexed documents. There is no such thing as fixed definition when triggering the precedence of the contract.