For questions related to segmentation or segments
The segmentation is an internal component of core Sitecore roles, and is used by the Content Management, xDB Processing, and xDB Reporting roles.
The segmentation engine is responsible for building a query from an
ISegmentDefinition or an
IContactSearchQueryFactory and returning a list of contacts or a count of contacts that match those criteria:
For more details refer to: https://doc.sitecore.com/en/developers/93/sitecore-experience-platform/segmentation-engine.html