I propose using <biblioset> only in a case where the <biblioentry> is for a resource that is part of a larger work. This could be a chapter in a book, an article in a journal, a book in a book series, or a conference paper in proceedings. When it is used, I suggest not using a separate <biblioset> for the main resource. Define the main resource directly under <biblioentry> and use <biblioset> to define the larger work (see Open Question 6). The child elements of <biblioset> are the same as for <biblioentry>. However, they may be interpreted differently (e.g., <volumenum>), and you can't nest <biblioset> elements.