The tab ONIX validation presents the special validation formats that you can add to ensure best practices' compliance with BTLF, BookNet Canada, BISG, BIC, APA , AMAZON and EDItEUR. See Specialized Formats section for more details. In the 2.1 section, the option "Validate ONIX 3.0 mandatory elements" when checked tells ONIXEDIT to also check for ONIX elements that are optional in 2.1 but mandatory in 3.0. Indeed, an ONIX 2.1 file that is completely valid may have errors when converted to ONIX 3.0. This option allow you to identify titles that may have such problems and fix it before you convert it to 3.0.