Access code application.capitalone.com
Capital one reviews your credit score to match you with credit card options that they feel you would make a good fit. You will find these offers with capital one cards like the capital one platinum MasterCard, and the capital one Quicksilver card.
The reservation number and the access code are basically tracking methods that capital one uses to streamline your application, and also track which marketing techniques are working. So there is no issues with using the access code, (or not using it) and it is legit, used by Capital One to track your application.