Open Packaging Conventions

Note: The older first edition of ECMA-376 makes an informative (i.e., non-normative) reference to the newer PKWARE Inc's ".ZIP File Format Specification" version 6.2.1 (2005).

The draft expired in 2009, the specified syntax is incompatible with the Internet Standard for URI schemes (STD 66, RFC 3986).

OPC also defines some naming conventions and an indirection method to allow position independence of binary and XML files in the ZIP archive.

Relationships are composed of four elements: OPC packages can store parts that contain any type of data (text, images, XML, binary, whatever).

If you open a ".rels" file in a text editor, you can view the actual XML markup that defines all the relationships targeted from that part.

A typical relationships file contains XML code like this: which defines two relations for the root package, the first one being considered as the root package (here for an early Microsoft XPS document, before it was standardized as Open XML Paper Specification within the openxmlformats collection), and the other one being used to reference an alternate form (here a thumbnail rendered image of the first page of the document).

This allows faster access and less object creation for clients and makes it easier for multiple processes to be working on the same document.