Letter identifiers in correspondance messages are (alpha-numeric) serial numbers which are unique for the sending system. The sender must ensure that the same number is not used twice. Maximum length is 14 characters. Derived from http://svn.medcom.dk/svn/releases/Standarder/Den%20gode%20korrespondance/XML/Dokumentation/XDIS91.pdf, see 'BrevNr'