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For many countries we have both XX and XX-citizens tags (e.g., usa and us-citizens), but I am not sure what the difference is. It seems to me that the idea is if you are an expat, or planning on being an expat, in the US, then you would use the usa and if you are a citizen of the US and are an expat, or planning on being an expat, in a country other than the US then you would use the us-citizens. If you are an expat in the US but planning on moving to another country then it seems odd to use the us-citizens since you are not. This also happens if you are a US citizen expat living in the UK looking to move back to the US. In this case it seems you would tag the question with both uk-citizens and usa, which seems wrong.
I think it is important to differentiate between the country you are, or planning on being, an expat in from your "home" country. I am just not sure the country you are a citizen of is the right description. The problem is that "citizen" has a specific meaning to an expat. A tag like xx-residents might be better, but it still has very specific connotations.