Data Holder using their own error URN

Question

CDS Error codes specifies a variety of error URNs to be used in responses to Accredited Data Recipients (ADRs). 

Can Data Holders (DHs) respond with their own URNs for cases that are not specified in the CDS? Alternatively, should DHs only use specified URLs and use urn:au-cds:error:cds-all:GeneralError/Expected or urn:au-cds:error:cds-all:GeneralError/Unexpected for unspecified cases?

Answer

It is permissible for a Data Holder (DH) to use their own URN but it must be presented as a custom error code, not presented in the URN field. See CDS Error codes. DHs implementing custom error codes must choose one of the fallback CDR codes for use in the "urn" field so that ADRs can reliably map custom error codes for each DH to a generalised CDR error.