I wonder if I REALLY meant to ask, who decides what the categories are? Aha! Excuse me for my senior moment. But truly, this IS important, because although you don’t get to decide the categories, you do have a say into which ones your book will go. So, here are some things to keep in mind when you are selecting categories:
1) Select as many categories as you are allowed: if you’re given 3 options, as BISAC allows, take them all (see http://bisg.org/page/JuvenileFiction just for fun!).
2) Try to be as diverse with those categories as possible. For instance, if your book is about a glum baby panda (yeah like THOSE exist) living at the local zoo, your BISAC categories shouldn’t all be under Animals. Think, what is the problem glum baby P is facing? How does s/he solve it? Use those clues to lead you to different categories altogether (seriously, look at http://bisg.org/page/JuvenileFiction).
3) Despite the belief that casting the widest net is the best strategy, I recently heard advice that we should be as specific about categorizing our book as possible. This is because, like the example above, if the only categories you’ve placed your book into is Animals: bears, Animals: baby animals, and Animals: Zoos, how many OTHER books do you think exist in these fields? Yup, A LOT. Ergo, the search will returns hundreds if not thousands of books. And you are no closer to selling a book.
4) If you’re truly split between or among a few categories, like if you HAD to put your book into one category container, WHAT box would that be? I've heard the argument that putting it into the hottest or trendiest category is a good strategy, because the hottest books are selling, and there are a lot of sales that happen just by association. In my opinion, it's worth trying.
5) Always remember that you are allowed to change these categories. See how you’re sales are going, ask anyone who tells you they have bought your book how they found it, and simply ask anyone who has read the book, how would YOU categorize it?
Now, I leave you with a game where I would love for you to post answers back. In the categories I have listed for Centipede Dragon, which three would YOU choose for my book?