J. McRee Elrod <[log in to unmask]> wrote:
A Soloka (are you the Alan Soroka formerly of UBC?) quoted:

>"Qualifier of title information to make it unique."

By "it" do they mean the title itself?

That definition is a slight paraphrase of the definition for the 222 $b, and by extension the 210 $b as well.  According to the script for "module.MBIB-2-BIBFRAME-Shared.xqy" on LC's BIBFRAME GitHub site (not sure how to get a shorter URL for that particular page) this BF property gets its due when converting records with the aforementioned subfields through the MARC-to-BF filter.

This 222:

 222 -0 $a Notes d'information et statistiques $b (Banque centrale des Etats de l'Afrique de l'Ouest)

gets converted into this (today, at any rate):

 <> a bf:Title ;
    bf:titleQualifier "(Banque centrale des Etats de l'Afrique de l'Ouest)" ;
    bf:titleValue "Notes d'information et statistiques" .

These are the nuts-and-bolts of the key title, linked from the Instance's bf:keyTitle property via that "title41" URL:

<> a bf:Instance,
        bf:Serial ;
    bf:keyTitle <> ;

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