When an audience is imported into Adobe Experience Platform from an external source (like a CSV upload or a partner destination), it is treated as an External Audience. Unlike audiences natively generated within AEP, external audiences are 'non-durable' by default regarding their metadata. This means that while the platform knows which profiles belong to that audience, the specific attributes associated with that external list (such as 'Destination.City') are not automatically ingested into the Real-Time Customer Profile as permanent attributes.

The 'challenges' mentioned occur because the Segment Builder requires attributes to be part of an XDM schema to perform complex cross-attribute filtering. Because the 'Destination.City' attribute exists only within the context of the external audience membership and is not linked to the unified profile schema, it cannot be used in a join with behavioral Event attributes.

Option A is incorrect because external audiences are limited in combination with any other dynamic criteria in the visual builder unless they are first converted into profile attributes. Option D is a potential workaround but does not explain the reason for the immediate failure. Option C is incorrect as the UI does support basic external audience selection. The fundamental issue is that external attributes are transient and not part of the XDM Profile store, thus they lack the relational 'glue' required for complex segmentation logic involving time-series events.