[Koha] Checkbox «Flagged» on subfield visibility framework setup

Hi! Even I read Koha manual <http://translate.koha-community.org/manual/master/en/catadmin.html#marcbibframeworks> (2.4.1.4. Edit Framework Subfields > Advanced constraint values > Visibility) and bug #9894, but it isn't clear to me what a "flagged" subfield means. Regards! Pablo