Posts can belong to a category, managed under Blog → Categories. Each category has a name, a URL slug, an optional description and a display order.
Nesting
Categories can be nested, meaning one category sits inside another, and that nesting shows up as a tree in the public blog sidebar. Viewing a parent category shows everything filed underneath it, not just posts filed directly against the parent.
Category pages
Every category gets its own page at /blog/category/{slug}/, listing its posts with the same layout as the main archive. These pages are included in your sitemap.
Deleting a category
Deleting a category doesn't delete its posts. They simply become uncategorised. Any sub-categories move up one level rather than disappearing with their parent.
Categories vs. documentation sections
Blog categories and documentation sections are separate lists, and they don't mix: a blog category never shows up in your docs sidebar, and vice versa. If you're organising help articles rather than posts, you want documentation sections.