There is a new custom attribute parameter "flag.mandatory".^[38]

Adding this parameter to mapped attributes (e.g. under [CadDocumentAttributes]) will cause them to be mapped during the initial model creation process (before geometry), as NX 2312 (or later) requires. Not setting this flag has no effect in previous NX versions, but starting with NX2312, if all mandatory attributes are not marked as such in the ifugnx.cfg file, model export will fail with the error "The attribute not found".

So see Section 2.2.8.4, “Common hints for Attribute Mapping sections” in ENTERPRISE 3Dfindit (Professional) - Administration.