Which automatically expands your reach based on your campaign’s conversion history to help get as many conversions as possible?
Ad Preview and Diagnosis tool
There are two types of Targeting optimization: “conservative” and “aggressive” (known also as Display Campaign Optimizer).
“Aggressive” targeting optimization relies on Google algorithms to predict, test, and optimize targeting based on your campaign’s historic data. If your campaign doesn’t have at least 15 conversions per month, you won’t be able to use “aggressive” targeting (Display Campaign Optimizer) in your ad groups.
Read more here: https://support.google.com/adwords/answer/190596?hl=en