- Thema uses the general term Crime to cover "detective" but provides for "mysteries" in a variety of contexts, while BISAC has a specific heading for FICTION / Crime, but combines Mystery and Detective.
- Thema places True Crime as a minor heading under a hierarchy of Literature / Biography & non-fiction prose, while BISAC provides True Crime as a major heading with subdivisions for Espionage and so on.
- Therefore, to reproduce a code to replace a subdivision of BISAC's True Crime, Thema codes may require use of a secondary subject code (like the non-fiction code for "Espionage & Secret Services") to add that meaning to its True Crime subject.
The point is simply that you should look at the book you are classifying and place it best within the subject system you are using. The translator can get you close but for a specific book one system may support options better suited to the specific title than the options in the other.
A book's CODES are hierarchical because retailers can take information from any part of the hierarchy, which makes the codes flexible. There's never a need to apply a high-level "fiction" code for a book once you've given it any subject within the "fiction" list. The specific code contains all the information a retailer needs to know about its higher levels.Another difference can be found in their respective "trees" which are represented in the CODES for Thema and in the LITERAL for BISAC. If that sounds confusing think of it as: You put the Thema code list in order by sorting on it's codes but you sort BISAC on its literal (aka description). In either system retailers can take the information provided and move up to a higher level.
BISAC by design is a less granular system than Thema (and that's one of its advantages). Thema offers more opportunities to use secondary subject to paint a picture about the book (and that's one of its advantages). The takeaway is each may offer opportunities to describe your specific book that the translator misses. The translator can get you close but for a specific book one system may support options better suited to the specific title than the options in the other.
A book's overall subject coding is NOT hierarchical