For each PDF/A-1 validation issue we try to explain what it means and, if possible how to resolve it. For a more technical explanation of the issue we quote the related section of the ISO 19005-1 PDF/A Specification.
Schema namespace URI: http://ns.adobe.com/xap/1.0/
Preferred schema namespace prefix: xmp
The preferred namespace prefix was xap in earlier versions of the XMP specification. Several applications still use xap for compatibility reasons.