About ad rotation settings
Ad rotation is the way we deliver your ads on both the Search Network and the Display Network. If you have multiple ads within an ad group, your ads will rotate because no more than one ad from your account can show at a time. Use the ad rotation setting to specify how often you'd like the ads in your ad group to be served relative to one another.
This article explains the two settings for ad rotation.
Optimize
This setting optimizes your ads for each individual auction using signals like keyword, search term, device, location, and more.
Powered by Google’s machine learning technology, the "Optimize" setting prioritizes ads that are expected to perform better than other ads within an ad group. All video campaigns are automatically optimized for views.
Tip:
How it works
As data is accumulated, ad serving will become weighted more heavily in favor of the ads that appear statistically likely to perform better. Ads expected to attract more clicks (and conversions if you’re using a Smart Bidding strategy) are delivered more often into the ad auction than other ads in the ad group. These ads show more often, resulting in higher ad-served percentages. However, it's possible for ad serving to remain relatively even within this setting, if ads in an ad group have similar performance or if the ad group does not receive a substantial amount of impressions and clicks for some period of time.
Keep in mind
Ad Rank considers landing page experience and ad relevance (among other factors) when determining your ad’s position and whether it will show at all. A more relevant ad with a better landing page experience could lead to overall better performance.
Rotate ads indefinitely
The "Rotate ads indefinitely" setting delivers your ads more evenly into the ad auction, but does so for an indefinite amount of time and does not optimize.
Because this option allows lower-performing ads to run as often as higher-performing ads for an indefinite period of time, this option is not recommended for most advertisers.
Keep in mind
With the "Rotate ads indefinitely" option, the percentage of impressions for ads served in the ad group will be more even than the other ad rotation options. However, because the quality of the ads differ, and quality is used to determine where an ad shows, or if it even shows at all, the percentage of impressions for each ad may not be perfectly even. An ad with high quality may appear on the first page of search results, while an ad with low quality may show on the second page of search results, which reaches a smaller number of users.
* Nguồn: Google