On the question Is it legal to go take my license plates off a car I sold, without realizing I should keep my plates? there has been some back-and-forth editing on the tags. Specifically texas and united-states were both added, and then united-states was removed again, with some discussion in the comments particularly this comment by Ryan M and this one by Strawberry.
Ryan M maintains that when a question appertains to only a single one of the US states it should use the tag for that state, and not the tag for the US as a whole. Strawberry responds that if one is using tags to filter questions, it is much more helpful to be able to exclude (or include I would add) all us-related posta by using a single tag than having to filter on 52 different ones (each state, DC, and US).
Now the tag description form the united-states tag reads:
For questions specific to the United States as a whole, or that span multiple state jurisdictions. If your question is related to a specific state then you should use that state's tag instead (or as well). (emphasis added)
This seems to permit, but not require, the use of the the united-states tag on questions about the law of a single state. Is, or is not, the use of both tags a good practice?
More generally, where a tag is broad, and another tag refers to a sub-section of its range (say eu and france or criminal-law and trial-procedure) is it a good practice to use both, or only the more restrictive sub-tag? Using the wider tag permits easier filtering and selection, but seems redundant on the question itself. I am honestly not sure, but it might be as well to have an agreed guideline to prevent future disputes, and tell people what they must do in searching and filtering.
I note that a related issue was discussed, but not settled, in Should the tag [united-kingdom] be removed, and replaced with its 4 countries?
There has been an objection to the term "subtag". I meant to indicate any tag whose agreed subject matter is a proper subset of the subject of another tag. Call it a "subset tag" if you like, and call the more inclusive tag a "superset tag".