I tag people that never fail to find something to argue about, obvious trolls, bad faith / sea lion conversationalists and so on. If I encounter them again and they show the same behaviour as a pattern, I fully block them.

I also use it for more fun stuff, like who is overly helpful, friendly or who recommended a good book. This way, if I see them again I get reminded of the positive interaction from before

edit: @OmegaLemmy@discuss.online you might want to replace "tag" with "label" or something, people think you mean username @tagging and don't read the body of your post