What are BISAC codes?

What does it stand for? Book Industry Standards and Communications
“The official definition of the term is an industry-approved “list of standard subjects designed for use in the book trade in the U.S. and English-Speaking Canada.”

There are 54 major sections, each with their own sub-sections
https://bisg.org/page/BISACFaQ

How do they differ from keywords?
These codes are used by retailers to ensure that your book is properly “shelved” and categorized. As in, your fiction book about space travel doesn’t end up under a reference or non-fiction space travel section. Keywords are used in your marketing copy so that your book shows up in search engines based on consumer searches.

You can usually select 2-3 of these when you upload your book in KDP or IngramSpark. It is important that you select the correct ones. In KDP these are called categories and for the most part Amazon has many categories beyond books so they don’t always align 100% and you can add up to 10 total through the secret method explained by Dave Chesson (https://www.youtube.com/watch?v=2GO-XNA0epE)

Infinite-Infinite
Fiction/Alternative History
Fiction/Science Fiction/ Time-Travel
Fiction/Science Fiction/ Action & Adventure

Or for Nailbiters:
Fiction/Science Fiction/ Post-Apocalyptic Science Fiction
Fiction/Science Fiction/ Alien Invasion Science Fiction
Fiction/Science Fiction/ First Contact Science Fiction (Books)
With this example of my two VERY DIFFERENT sci-fi books, I don’t want them to be in the same category or have the same BISAC code. The person who likes sci-fi action & adventure, it not likely to enjoy the post-apocalyptic alien-invasion story.

I’ve seen a fictional book ranked as a top seller in a non-fiction real estate investment category. Like, great that you’re highly ranked in that category, but anyone who goes looking for the best books on how to invest in real estate and buys that book because it is ranked high in that category is going to be VERY confused when they read it and likely leave a bad review. Which “hacking” categories is a strategy that some authors use to boost their rankings on Amazon or other retailers, you still need to be smart about it. Make sure your book aligns with the BISAC code you have assigned to it.