Problem UNIMARC 100 $a/17-19 Target Audience Code It is not possible to use the "#" symbol. Unused positions are filled with space characters "#". For example, when you need to use "x##". https://cdn.ifla.org/wp-content/uploads/files/assets/uca/unimarc_updates/BIBLIOGRAPHIC/u_b_100_update2020_online_final.pdf