CCDC 24 is the document the ExAC reaches for when a question asks "which form should the architect issue" or "what information should that form contain." Examitect's ExAC study plan lists it as a primary reference for two Bidding and Contract Negotiations objectives (procedures for awarding a contract; evaluating bids) and two Construction Phase Office Functions objectives (the office-function tasks of construction administration; demonstrating the use of administration forms).
The language of the exam also tracks the language of CCDC 24. Terms like Proposed Change, Change Order, Change Directive, Supplemental Instruction, Application for Payment, and Certificate for Payment come straight from the model forms in this guide and the General Conditions in CCDC 2. Knowing which instrument fits a given situation is one of the more reliable ways to move from a 60-percent guess to a confident answer on Section 4 questions.
If you only had time to study one CCDC document beyond CCDC 2 itself, this would be it.