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If used, the national standardized reference has to be filled in this field. Reference must start and end with three "+"-character. Between plusses there has to be 12 digits.
Example reference: <Ref>+++312401398067+++</Ref>
The Reference contains a 12 digit structured communication, with the last 2 digits as check digits (modulo 97) of the first 10 digits; but if the result is 0, then the check digits are 97.