Assembly: WindowsBase (in WindowsBase.dll)
When is Internal, the TargetUri MUST be a relative reference. For a package-level relationship, the relative reference of the TargetUri is resolved relative to the pack URI of the source package that owns and contains the relationship. (Example source package pack URI: "pack://http:,,www.adatum.com,files,datapak.package/".) For a part-level relationship, the TargetUri is resolved relative to the source part that owns the relationship (TargetUri is interpreted relative to SourceUri).
When is External, the TargetUri can be either a relative reference or a fully qualified URI. If the TargetUri is a relative reference, the reference is interpreted as relative to the location of the Package that contains this relationship.
For more information about see Section 1.3.3 "Relationship Markup" of the Open Packaging Conventions specification available for download at http://go.microsoft.com/fwlink/?LinkID=71255.