2.2.2 Relationships (Defined by OPC)

Topic Version1Published09/15/2014
For StandardEPC v1.0

OPC provides mechanisms for defining relationships between the package parts, and external resources. The mechanism can be used for both internal parts and external resources, meaning that relationships can actually point outside the package, which includes a file on a Web or file server, a Web service address, or almost anything that can be described by a URI.

Each folder of the package (including the root), has a special folder with the reserved name “_rels”. This folder contains files whose names are made up of any file name in the parent folder, with the reserved extension “.rels” appended to it.

The relationships for the package as a whole are defined in the file /_rels/.rels. (This is commonly where the location of the core properties file is stored.) The following table explains the key components of the .rels file.




A unique name for the relationship. It must be unique within the context of the overall package


The file or resource to which the relationship refers. It can point to a part inside the package or to an external resource.


Indicates the kind of relationship that exists.