BISAC Subject Headings
The BISAC Subject Headings are a method to classify books that is geared towards bookstores. It is mainly used by the Northern American booktrade, and online sellers like Barnes & Noble, Amazon and Baker & Taylor. The Book Industry Study Group maintains the BISAC system.
BISAC, an acronym for Book Industry Standards and Communications, classifies all works by topics. All topics and sub-topics are ordered alphabetically, which also encompasses works of fiction. The BISAC's numbering scheme allows for a later introduction of new topics between already established ones.
Books can fall into several categories. In this case, BISG recommends that no more than three categories should be chosen for a single book, and the main category should be the one that best describes the book's contents.