*OK, they aren’t really secret, but it takes special keywords to get placed into them.
Amazon has a variety of 3rd and 4th level categories that aren’t listed in the category selection tool when you are adding/editing your book in KDP. Often, it is helpful to have your book in a smaller category so that it can get higher exposure and hopefully more sales. So how do you find those categories to put your book into them, if they aren’t available for selecting. You need to add secret keyword!
To put your Christmas romance book into Romance/Holidays – you’ll need to add one of these keywords [christmas, thanksgiving, valentine, halloween, new year]. (See list)
To put your kids book into Children’s Age Range/ Ages 3-5 you’ll need to add the keyword [preschool] (see here)
To put your mystery crime novel into the Mystery, Thriller & Suspense/Crime Fiction/Heist sub-genre, you’ll need to add one of these keywords [heist, robbery, thief, theft] (see here)
Oncce you’ve done this, wait about an hour after your book change is accepted (you know – the 4-12 hour wait) and then if your book is ranking, you should also see it ranking in that category too (if you have enough sales). It’s often easier to rank higher in a smaller category due to a few reasons: Less people have figured out how to put it into the category (Now YOU know how) and generally that means less competition. Second, some people when searching will actually, truly search the smaller categories for a book more closer to what they are looking for. And now they will more easily find you.
Here is the main link describing how to use categories for KDP: https://kdp.amazon.com/help?topicId=A2EZES9JAJ6H02
At the bottom of THIS page is a list of categories that have secret keywords and what they are: https://kdp.amazon.com/help?topicId=A200PDGPEIQX41 (bottom of page for category lists)
Just another way to get more visibility and help generate more sales within Amazon.