Used by the book industry and in a few cases, libraries.
BISAC stands for Book Industry Standards and Communications.
From the Book Industry Study Group (BISG):
This year’s edition focuses on the addition of the Young Adult Fiction (YAF) and Young Adult Nonfiction (YAN) sections for the young adult audience (ages 12-18), and an adjustment on the age group that the Juvenile Fiction (JUV) and Juvenile Nonfiction (JNF) sections cover, now preschool to 6th grade (ages 0-11). More than 500 new codes are included, the majority of which are within these two new sections.
From the 2015 Edition Info Page:
BISAC Subject Headings describe the topical content of a book and do not cover non-content-oriented ways of grouping titles (such as “gift books”). Other fields in BISAC communications formats (language, audience, age range, grade range, format/binding/packaging, supplementary format/binding/packaging, merchandising themes) are used to describe a book in ways not covered by the Subject Headings list. Since Merchandising Themes are closely related to Subject Headings, the Themes and codes used in this field are included at the end of the Subject Headings package (along with usage instructions).
The BISAC Subject Headings are available for purchase but several free tools are available for all users.