An advertising segment in Google Analytics is the list of users in Google Analytics that are synced with Google Advertising products for use in remarketing and ads personalization.
Your Google Analytics advertising segments include users collected by Google Analytics, and sent to advertising products for use in ads personalization. These can be viewed, including their ads personalization eligible list sizes, in the Advertising segments panel of the Google Analytics Advertising section. These sizes match what is shown in the Google Ads Audience Manager.
Size of your Google Analytics advertising segment
The requirement to serve your ads varies based on the target network. Learn more about advertising segment sizes.