Thema is an internationally-developed set of subject codes meant to serve publishing's global trade needs. Whereas North America uses BISAC, the UK uses BIC, English-speaking countries use both BISAC and BIC, France uses CLIL, and Germany uses WGS. Thema is designed to be a sort of lingua franca to which all these schemas can be mapped. This way, a publisher using BISAC codes, having mapped them to Thema codes, can send out information in a way that a recipient using CLIL can make use of.
Thema is multi-lingual, and its implementation reduces the need for duplication of work in similar workflows. It is heavily in use in the EU, of course, because inter-lingual communication is a daily need there. It is gaining traction in English-language countries who have to work with both BIC and BISAC. In the US, implementation has not gone as far - that will happen when US publishers are more affected by global market communication requirements than they are now.