any number of asset records between systems. The ""pbcoreInstantiationDocument" provides a metadata container for physical or digital instantiations for which no descriptive metadata exists, which was disallowed in previous versions of PBCore. Another significant change in PBCore 2.0 was the addition of optional attributes for many elements, including source, ref, version, annotation, startTime, endTime, and timeAnnotation. The source attribute allows for the specification of a source of subject terms and other values, while the ref attribute may contain a URI for the subject term itself. The version and annotation attributes allow for the inclusion of versioning for the value, and narrative information intended to clarify the nature of the data used in the element. The startTime, endTime, and timeAnnotation attributes allow for the inclusion of time-based metadata, for example the location in the media timeline when a subject, name, or location was relevant. A third major change to the schema was the addition of pbcorePart as an optional element of the root element pbcoreDescriptionDocument, and recursively another pbcorePart element, which allows for defining segments, stories, or episodes of a larger media work. Current developments As part of the American Archive of Public Broadcasting initiative, PBCore entered a new phase of development in 2014. The American Archive team established a PBCore Advisory Subcommittee of the Association of Moving Image Archivists to reassess the schema, continue outreach to users and potential adopters, create resources for the PBCore community and improve the website. Content classes The PBCore standard is built on the foundation of the Dublin Core (ISO 15836), an international standard for resource discovery, and has been reviewed, though not endorsed, by the Dublin Core Metadata Initiative Usage Board. PBCore extends Dublin Core by adding a number of elements specific to audiovisual (AV) assets. These AV assets can be physical analog media items, or digital media objects. PBCore provides a standard for cataloging and describing media objects in three general ways: Intellectual Content provides descriptive metadata including Title, Subject, Description, and Genre. The Intellectual Content for the asset contains no metadata concerning the physical or digital representation of the asset. Intellectual Property provides metadata concerning the creator, distributor, and publisher of the AV asset, along with rights information about its use. Instantiation contains all technical metadata about the physical or digital representation of the AV asset. This can include format, media type, duration, file size, data rate, aspect ratio, frame rate, and