[I]f you choose an opt-out cookie, DoubleClick is unable to recognize your browser from one visit to the next, and you may therefore notice that you receive the same ad multiple times.
What happens if I choose to opt-out?
The system will assign you the following cookie:
id=OPT_OUT
instead of a cookie that contains a unique string of numbers
By receiving this cookie, DoubleClick’s systems will know that you have opted out and will not to attempt to assign other cookies in the future. Since the DoubleClick opt-out cookie does not contain a unique user number, it does not allow us to identify your browser uniquely.