Audiences define sets of Profiles which are members of the Audience's defined Segments. An Audiences member Profiles may be sent to Destinations to keep external marketing platforms in sync with an identical set of Profiles.
Audiences are defined by a list of Segments that are used to determine whether a Profile should be included in the Audience.
Audiences are dynamic, and are calculated at the moment they're accessed. The set of Profiles included in the Audiece is determined by unioning the resulting Profile IDs from the collection of Segments in the Audience.
When sending Audiences to external systems, membership is evaluated at the time of the sync. Different Audience Destinations may update in different ways, so please refer to the documentation for each Audience Destination to understand those differences.