BISAC codes, as I discovered while preparing to publish Restoration Day, are basically subdivisions of genre. And sub-subdivisions, and so forth. Not content with classifying a book as Fiction, they will go further and classify it as, say, Mystery & Detective. And then break it down further: Hard-Boiled? Or is it Cozy? And if Cozy, then Cats & Dogs, Crafts, Culinary, or General?
Old-Fashioned Fruitcake