If there is exactly one <relatedItem type="host"> element of an element of <genre authority="marcgt">book</genre>, currently there is no way to distinguish between multivolume monograph or series, i.e., whether this was mapped from a biblatex "maintitle" or "series" field.
Thus, <relatedItem type="host"> elements require disambiguation, and I’d still suggest doing this by including <genre authority="marcgt">multivolume monograph</genre> or <genre authority="marcgt">series</genre>.
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If there is exactly one
<relatedItem type="host">element of an element of<genre authority="marcgt">book</genre>, currently there is no way to distinguish between multivolume monograph or series, i.e., whether this was mapped from a biblatex "maintitle" or "series" field.Thus,
<relatedItem type="host">elements require disambiguation, and I’d still suggest doing this by including<genre authority="marcgt">multivolume monograph</genre>or<genre authority="marcgt">series</genre>.