This TechNote explains how to construct an XMP extension schema for PDF/A-1. It complements TechNote 0008: Predefined XMP Properties in PDF/A-1. Both TechNotes should be used in combination as follows:
In addition to talking about XMP extension schemas for PDF/A-1 this TechNote includes various general notes on XMP and Acrobat. These may be useful when working with XMP extension schemas.
Extension schemas can also add properties to the predefined XMP schemas. In order to add new properties to one of the predefined schemas you must also create an extension schema according to this TechNote. It is recommend to not include descriptions of predefined properties in such an extension schema container schema. However, if predefined properties are included in an extension schema it is recommended that PDF/A validators ignore such property descriptions.
Note: The term schema in the context of XMP metadata for PDF/A-1 is unrelated to XML schema definitions (.xsd).