The Id property string is unique for all relationships owned by the SourceUri package or part.
The Id is specified in the call to the Package.CreateRelationship or PackagePart.CreateRelationship method that created the relationship. After the relationship is created, the Id cannot be changed.
The Id property string is a valid XML identifier. The Id type is xsd:ID and must follow the naming conventions prescribed in the XML Schema Part 2: Datatypes specification (see http://www.w3.org/TR/xmlschema-2/#ID).