In Experience Data Model (XDM), a union schema is a unified view of all schemas belonging to the same class that have also been enabled for Real-time Customer Profile. The union schema is leveraged by Profile in order to construct a complete representation of a customer from disparate experience data.
In some cases, you may be ingesting data that is not necessarily part of a profile, but is nonetheless related to the profile. An example of this kind of data would be a “favorite hotel” field for a customer. Since the attributes of a person’s favorite hotel are not attributes of the person themselves, a hotel is best represented by a separate schema based on a custom class rather than XDM Individual Profile.
Since union schemas are only based on schemas that share the same class, simply enabling the “Hotels” schema for use in Profile will not include its fields union schema for XDM Individual Profile. Instead, you must define a relationship between “Hotels” and another schema belonging to the union. This involves defining a relationship field in a source schema that references the primary identity of a destination schema.
For detailed steps on defining a relationship between two schemas in the Adobe Experience Platform UI, see the relationship UI tutorial.