Thema - thoth-pub/thoth GitHub Wiki

‘Thema is a multilingual subject category scheme designed to meet the needs of publishers, retailers and trade intermediaries in all sectors of the global book publishing business. It is managed by EDItEUR and an international group of stakeholders, and is free to use. […] The scheme is free of charge for all to use, subject to EDItEUR’s normal, highly-permissive license. No registration, fee or membership is required to implement and use Thema.

Thema subject categories: https://ns.editeur.org/thema/en

Convertor from BISAC to THEMA https://www.booknetcanada.ca/blog/2014/4/24/introducing-bncs-bisac-to-thema-translator.html