3.4.1 Rules for Specifying Relationships in EPC

Observe the following rules and guidelines when specifying relationships in EPC:

• If a data object is broken into many files, every file (part) must be linked together with _rels. Example: WITSML contains very large log files, which are often chunked into several smaller files.
• Every XML data object that contains an element of type DataObjectReference is required to have that relationship documented within the OPC relationship system.
• The relationship file must define all the forward and backward relationships of the XML data object. So for every relationship between two XML data objects there are two .rels files; one for the source object and one for the destination object.
• Relationships to non-XML data objects can be documented but are optional.
• If an XML element contained in an EPC file is involved in one conceptual relationship, then the EPC file part must have an associated rels file. These XML files can be from any of the Energistics data-exchange standards including: RESQML, WITSML, and PRODML. Rels files for other EPC file parts (jpg, pdf, etc.) are optional.
• The ID of the relationships follows the OPC format: it is an XML id. It must start with a letter or an underscore (not a digit). It is acceptable for the ID to always start with an underscore.

• The Type of a relationship shall be one of the Energistics defined types as given in Section 3.4.2 Relationship Types and Namespaces in EPC