If you, dear author, are uploading your title to IngramSpark or Amazon KDP, you will soon encounter book category codes. Getting these right is quite important and deserves some consideration, rather than speeding on to the next bit of metadata.
IngramSpark goes into some detail on the subject of BISAC codes, and there is a lot more here.
The source site for Thema is not very user friendly, but it does explain the methodology for this system. The Australian Publishers’ Association has this to say:
What’s in it for Australian publishers?
The quick answer? Increased discoverability and sales.
Thema will ground your book in a whole new layer of discoverability that works for retailers and consumers alike. The wants of readers are constantly changing, and if you’re providing what the Australian reader wants but they cannot find it, you’re missing out on sales.
Reedsy has quite a comprehensive guide to the Amazon KDP category codes. Amazon’s own guide is here.