BISAC is an acronym for Book Industry Standards and Communications.
The BISAC code is a nine-character alphanumeric code. It tells retailers what your book is all about and in what section (fiction, education, design, pets, nature, music) it will fit best.
To choose your BISAC subject headings go to https://www.bisg.org/bisac/complete-bisac-subject-headings-2015-edition.