Immigration status.
That came about a few years ago when they were closing some loopholes. The basic explanation is that a British post will not normally accept settlement applications unless the the applicant is a citizen of the consular district they are applying in *OR* unless they are ordinarily resident in that consular district. If they are applying on the latter of these, they must show proof that they are ordinarily resident in that country, and it needs to agree with the UK's definition of ordinarily resident - which is rather thorny.
Without that rule in place, everybody would get married, then hop over to the Netherlands or France and apply for settlement at the British consulate and get back to the UK possibly in the same day. And they can't be doing with that. Also, somebody from a visa national country like Peru, for example, could enter the US as a tourist, get married to a Brit, and go to the consulate in LA and apply for settlement in the UK. In that case, the consulate would not be plugged in to the Peruvian police records to check the person out; and they would have to make a referral anyway.
BUT if that Peruvian national had a US green card, then it's ok because they could show that, although they were not a US citizen, they were ordinarily resident in the US under the UK's definition of "ordinarily resident". And it would be silly to make them leave their home just to apply for a visa.
So it makes sense, and that's why they have the question on the form. In your case, you are US citizen applying inside the US. So that question does not apply to you. You would put "Not applicable" or "N/A" or whatever is consistent with how you indicate that the question does not apply. I draw a big diagonal line from one corner of the box to the other, but it's up to you. Best not to leave anything blank.
That should, hopefully, be helpful in filling out your form. You don't need a note from your parents
I can now see where this would be confusing to non-visa nationals, and I can sympathize that this information does not appear on their website. They probably think that since it does not apply, that nobody will worry about it. And it's a safe bet for sure that a visa national would *absolutely* know what's going on. But there's no reason they couldn't put a note on their site about it.
