A COR is the person the contracting officer appoints to monitor your day-to-day performance, the technical work, deliverables, invoices, on a specific contract, and is usually your main point of contact once you are actually performing. What a COR cannot do is change your price, scope, or schedule; only the contracting officer holds that authority, no matter how reasonable the COR's request sounds. Treat anything that looks like new work or a schedule change as a signal to loop in the contracting officer, not as instructions you can just start executing.

In a sentence

"Direct all technical questions to the COR identified in the contract; contract changes must go through the contracting officer." A COR can tell you whether your deliverable met the requirement, but cannot promise you extra money or extra time for it.