Discourse supports as a fundamental design notion a taxonomy of Categories and Topics. There can be many Categories, each with many Topics.
We don’t need a large number of categories. You can view the list. Most posts will go in General. There’s a category For Sale for commercial posts to which we have no objection within reason.
There are categories for site feedback and site usage. Please post any comments about the site in the appropriate category, not in General.
All posts are required to have a category, You cannot make a post without one. This is slightly different to some Discourse sites where uncategorised posts go in the marvellous philosophically paradoxical named Uncategorised category. i have had to disable that for reasoin related to running this forum in parallel as a mailing list as well, about which more in another topic about mailing list usage.