What happens when a user meets the frequency cap for an ad?
- Ads stop serving to that user
- Ads serve at a lower CPM
- Another ad or a default ad is served
- The placement stops serving to that user
Default ads serve when no other ads can serve. All non-default ads use frequency capping. This feature relies on the use of DoubleClick cookies in order to function, so if a user has disabled cookies within their browser, frequency capped ads will not be eligible to serve.
Read more here: https://support.google.com/dcm/answer/2828553