Yandex.Audience segments can be used to create ad campaigns in Yandex.Direct and Yandex.Display.
These segments are updated regularly. New anonymous identifiers are automatically added to them and old ones are deleted. This way segments only include anonymous identifiers that were active in the last 30 days. Anonymous identifers become active when someone uses their app or browser. Of course users can change browsers, re-install apps and acquire new devices, so segment coverage can change over time.
You can also use the API to manage your Yandex.Audience segments and access settings.