Serialization of XMP
The abstract XMP data model needs a concrete representation, when it is stored or embedded into a file. As serialization format, a subset of the W3C RDF/XML syntax is most commonly used. It is a syntax to express a Resource Description Framework graph in XML. There are different equivalent ways to serialize the same XMP packet in RDF/XML.
The most common metadata tags recorded in XMP data are those from the Dublin Core Metadata Initiative, which include things like title, description, creator, and so on. The standard is designed to be extensible, allowing users to add their own custom types of metadata into the XMP data. XMP generally does not allow binary data types to be embedded. This means that any binary data one wants to carry in XMP, such as thumbnail images, must be encoded in some XML-friendly format, such as Base64.
XMP metadata can describe a document as a whole (the "main" metadata), but can also describe parts of a document, such as pages or included images. This architecture makes it possible to retain authorship and rights information about, for example, images included in a published document. Similarly, it permits documents created from several smaller documents to retain the original metadata associated with the parts.
Read more about this topic: Extensible Metadata Platform