We use tags to make it easier to find posts on certain topics. This takes two things:
We allow anyone to add any tag to a post (point #1). We also allow anyone to upvote or downvote the tags on a post.
If a tag on a post falls to zero karma, it is removed from the post (point #2).
If a tag on a post gains additional karma, that post will be seen as more relevant for the tag, and will move up the list of posts on the tag page.
When in doubt, please err on the side of adding a tag! Other users can downvote it if they don’t think it should be attached, and the more people suggest tags in the first place, the better-organized the Forum will be.