Yandex.Audience segments can be used to create ad campaigns in Yandex.Direct and Yandex.Display. Segments consist of cookies found in actual users' mobile apps and browsers. Coverage refers to the overall number of cookies in a segment.
Segments are updated weekly. New cookies are automatically added to them and old ones are deleted. This way segments only include cookies that were active in the last 30 days. Cookies are activated when a person 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.