What is wrong, with the offending values quoted.
OptionalnavNAV's own description of the fault, in the requested language.
nav for a code NAV defines, local for one of ours.
Dotted path to the offending value, e.g. invoiceLines.line.2.quantity.
The fault code. Where NAV defines one it is used verbatim, so a local failure names exactly what the service would have said, and the compiler checks it against the generated catalogue.