The source – support.google.com
Auto-optimization is Google's experimentation with ad format settings. Systems automatically apply improvements that can increase revenue or improve user experience.
The new tweaks are:
1. Selecting a share of traffic. You can select the percentage of users who will participate in the experiments. For content it is recommended to choose not less than 50% of the traffic, and for search – not less than 5%.
2. Lock style. You can remove an ad style so that auto-optimization doesn't experiment with it. Works only for search.
3. Block experiment. Advertisers can remove the types of optimization experiments that don't need to be performed on the site.