checkmarc.pl L54 # checks biblio.biblionumber and biblioitem.biblioitemnumber (same tag and tab=-1) work in MARC21 as they are in 999 But in UNIMARC base data they are different tag biblio.biblionumber 001 @ biblioitems.biblioitemnumber 090 a No idea if sample data is correct but Tamil's UNIMARC Koha demo have this so it's should be good.
I just checked in a UNIMARC installation and there is no longer any warning about biblionumber and biblioitemnumber.