The thesaurus plugin creates a new authority record only when linked to UNIMARC bibliographic field 700. No authorities get created when the plugin is linked to fields 500, 606, 701, 702, 71x. BiblioAddsAuthorities is also turned on.
What is the "Thesaurus plugin"?
I think what is meant is the authority-linking-plugin possibly? I am not sure how this works for UNIMARC.
I wonder if this has something to do with MARC21 vs. UNIMARC. 700 is an authority controlled field in MARC21, the others are not.