If metadata extensions to PBCore are assigned to a media item using the descriptor extension, and the terms used are derived from an external authority or metadata scheme, use extensionAuthorityUsed to identify whose metadata extensions are being used.
PBCore recommends that the organizational authority responsible for the existence, maintenance, and sustainability of a particular metadata descriptor be identified using the Namespace or Registration Authority associated with that metadata element. Best practice is to use an actual URL or URI.
There are several sources that discuss the unique identification of a particular metadata element within a metadata schema.
XML schemas to support the Guidelines for implementing Dublin Core in XML--Recommendation:
To see the XML interpretation for examples of the elements extension and extensionAuthorityUsed, according to the PBCore XML Schema, refer to our web page of PBCore Metadata Examples and view the XML rendition of the descriptions for those examples employing extensions (examples 4-10).
Apply once within its container, pbcoreExtension (which itself can be applied multiple times for each additional metadata description outside of PBCore for a media item)