Categories differ between Primary Marketplaces (for example, .ca, .com, and .co.uk) and between formats (e-book and paperback / hardback).
You can select up to three categories for each version of your book. It’s recommended to use all three in order to increase your sales opportunities. The right categories will help readers discover your book through searching and browsing.
Invest some time in reviewing the available categories (there are thousands of options!) and select the three that best align with your book’s content and audience.
How to select categories:
Click Select One to find an overall category.
After selecting a main category, you will be prompted to select a subcategory (or two), and then a placement within that category.
Once you have chosen the first category, click Add another category to repeat the process.
To delete a selected category, click Remove.
When you have chosen all of your categories, click Save categories.
Categories can be changed at a later date, even after publication.
For paperbacks, there are two additional checkboxes you should only use if they apply to your book:
Low content books – for books with minimal or no content on the interior pages (sketchbooks, notebooks etc).
Large print book – for books that use a 16-point (or bigger) font.
More information about categories can be found here: https://kdp.amazon.com/en_US/help/topic/G200652170#unavailable