The following conformance rule was added to explicitly spell out the behavior when encountering a relationship in an XPS Document whose Target references a part that does not exist in the package.

Producers that generate a relationship MUST include the target part in the XPS Document for any of the following relationship types: DiscardControl, DocumentStructure, PrintTicket, Required Resource, Restricted Font, StartPart, StoryFragments, and Thumbnail. Consumers that access the target part of any relationship with one of these relationship types MUST generate an error if the part is not included in the XPS Document [M2.77].