Categories are used exclusively with the API to help segment discussions in queries. A category is a subdivision of a forum (shortname). In order for a thread to be assigned to that category, the forum must match.
Categories may make API queries slower if there are too many in a forum. Limiting each site to 15 categories will avoid this.
A thread can only be assigned to a single category.
Categories must be created via the API.
Use the categories/create endpoint to create a new category for your forum via the API. Save the category id for later.
Assigning categories to threads
You can assign categories to discussions one of three ways:
Discussions editor - Use this for threads that have already been created, or to change the category later.