Not all value types, defined in XMP Specification, are included in org.apache.padaf.xmpbox.parser.NSMapping. Thus its method isBasic() returns false on such types, like Choice or Real.
If PDF/A file includes custom metadata and its extension schema description with such types, PDF/A validation using Preflight fails with error like this:
"7.6 : Error on MetaData, ValueType 'closed Choice of Text' is unknown. no declaration found in this schema"
These value types are not included and regarded in NSMapping class:
• Basic value types: (open and closed) Choice, Dimensions, Locale, MIMEType, ProperName, Real, Thumbnail, XPath
• Media Management value types: AgentName, RenditionClass, ResourceEvent, ResourceRef, Version
• Basic Job/Workflow value type: Job
• EXIF schema value types: Flash, CFAPattern, DeviceSettings, GPSCoordinate, OECF/SFR, Rational
Note, that XPath type in NSMapping is included only in conjunction with bag, but spelled incorrectly as "bag Xpath".