This specification is one of a family of related specifications that compose EPUB 3, the third major revision of an interchange and delivery format for digital publications based on XML and Web Standards.It is meant to be read and understood in concert with the other specifications that make up EPUB 3: A resource that contains content or instructions that contribute to the logic and rendering of the EPUB Publication.The ISBN is an example of a manifestation identifier, and is shared by all instances of that manifestation.All instances of a manifestation need not be bit-for-bit identical, as minor corrections or revisions are not judged to create a new manifestation or work. The Unique Identifier may be shared by one or many Manifestations of the same work that conform to the EPUB standard and embody the same content, where the differences between the Manifestations are limited to those changes that take account of differences between EPUB Reading Systems (and which themselves may require changes in the ISBN). The Package Identifier allows any instance of an EPUB Publication to be compared against another to determine if they are identical, different versions of the same Manifestation, or unrelated.The Unique Identifier is less granular than the ISBN. Refer to Package Identifier for more information.].All sections of this specification are normative except where identified by the informative status label "This section is informative".
It must process the Package Document as defined in Package Document — Reading System Conformance, and honor all presentation logic expressed through the Package Document (e.g., the reading order, fallback chains and bindings).] video codecs, but this is not a conformance requirement; a Reading System may support no video codecs at all.
The following list summarizes the information a Package Document contains: This specification makes a distinction between the Unique Identifier for an EPUB Publication and the identifier that uniquely identifies a specific version of it (i.e., to be able to differentiate EPUB Publications containing different versions of the same Manifestation).
Two copies of an EPUB that are bit-for-bit identical are the same version and must retain the same last modified date.
Changes between versions may include minor typographic or markup corrections, without affecting the Unique Identifier.
Significant revisions to the content that result in a new edition require a change of the Unique Identifier.