I've noticed with the latest fortnightly challenge that the majority of the puzzles being created are actually chess puzzles that are additionally being tagged with board-games (the challenge topic)*. Now, I don't have an issue with this per se, but it was my impression that you should tag things with the best fitting tag, rather than just all appropriate tags.
There are lots of examples of these tag subsets (for example, you could argue: liars ⊂ logical-deduction; rebus ⊂ visual; probability ⊂ mathematics; etc), which naturally arises when there is a broad category, having one subcategory significantly more common than others, and thus is worthy of its own tag.
To me, the logical usage that results, would be to use the most specific tag that is applicable and not the broader one, only using the broader one when it explicitly is not the specific one. Thus a chess problem should be tagged chess, but a backgammon problem should be tagged board-games, and a puzzle that somehow incorporates both would be tagged chess+board-games.
Now, I'm not proposing anyone re-tag anything, I'm just wondering what the consensus is on the best/most useful tagging approach. Ultimately, I guess my question is: When tagging a question, should you only select the most specific tags, or should you include the broader tags as well?
*As of writing, there are 6 entries to the fortnightly challenge, with 5 of them tagged chess. Notably two of them aren't actually tagged additionally with board-games, which I don't have a problem with, and is arguably the more correct thing to do (i.e. allow board game related sub-tags to be included in the fortnightly challenge, without requiring actually tagging with the broader and redundant tag).