A quick overview of what you used to do, to what you do now
Yesterday Amazon KDP made an announcement about the way we select categories for our books.
The official announcement on the Amazon KDP category page: https://kdp.amazon.com/en_US/help/topic/G200652170
Previously, when you uploaded your book, you were able to select 2 categories for it to appear under. Then, you could add your book to up to 8 more categories if you emailed KDP support and asked them to update it manually.
The issue with this way of getting your books into particular categories meant you had to wait a random amount of time to be added to more categories, and they often got it wrong, so you needed to re-email KDP support and let them know which categories were wrong and what to change them too again.
Now, they’ve streamlined, but restricted, the process somewhat.
When you upload your book, you are able to choose up to 3 categories. You are no longer able to email KDP support asking them to add your book to any extra.
Make sure you do your research, and work out which three categories will benefit your book sales the most.