Smart contractors always request a REA before they request a claim or CDA. A REA is more informal and gives the Contracting Officer more "Wiggle Room". Once the Contracting Officer issues a formal decision then it is usually set in stone. A REA gives a contractor a chance to discuss the issue with the CO.
A REA can always be changed to a formal claim. The the costs of pursuing a REA are allowable under the FAR whereas a claim is not.
I could go on in excrutiatingly boring detail. If you want to know any specifics PM me.