Updating this post with some recent feedback from ServiceNow support:
kb_category is a reference field tied to the Knowledge Category hierarchy, while category is a separate string field with documentation text describing it as a lower level of article categorization where topics can have multiple categories.
Treat kb_category as the supported field for linking a Knowledge article to a record in the Knowledge Category hierarchy. The expected outcome is that an article is assigned to a category or subcategory from that knowledge base taxonomy. Treat the kb_category table as the place where the category hierarchy is maintained for each knowledge base.
Treat category as a distinct field on kb_knowledge, not an alias of kb_category. Retrieved metadata shows it is a string field, and its English field documentation describes it as "Lower level of article categorization - Topics can have multiple Categories." Because the retrieved metadata confirms the field exists separately but does not fully prove current product behavior beyond that documentation text, use kb_category as the authoritative article-to-category linkage when you need supported knowledge category classification behavior.
If your goal is article classification within a knowledge base, use kb_category and the Knowledge Category hierarchy.
If you are evaluating customizations or legacy usage of the category field, review where that field is referenced in your implementation before relying on it for active classification logic.
