In this question, it came up that the U.S. census, in its community survey, asked people to tell not only about their ethnicity ("race"), but also about the nationality of their ancestors. The question asked for a voluntary identification, and multiple nationalities could be listed (question 13):
What is this person’s ancestry or ethnic origin?
(For example: Italian, Jamaican, African Am., Cambodian, Cape Verdean, Norwegian, Dominican, French Canadian, Haitian, Korean, Lebanese, Polish, Nigerian, Mexican, Taiwanese, Ukrainian, and so on.)
This was beside much more objective questions, like "were you born in the U.S.?", "if not, which country were you born in?". Another typical question that would be asked in my country (Germany) would be "which country were your parents born in?". The purpose of this line of questioning in Europe would be as a tool for what is called integration strategies. It stems from the notion that these people with a migration background are either not fully self-identifying with the country they live in or are not perceived/alleged to be a full part of that country - and this is seen as a call for political action.
The question of "ancestry" in contrast seems to refer to a migration that might lie far in the past - even centuries or tens of generations back, if the askee wants to identify him/herself with that origin. It makes me wonder who is interested in that sort of self-identification, and for what purpose? Objectively, there could be dozens of nations one's ancestors might have come from. So I suspect most people would only name some of them, leaving others out of consideration. (because they were women, slaves, outsiders, ...?)
What sort of political discussion would this information be used in? As the census bureau says,
the survey generates data that help determine how more than $675 billion in federal and state funds are distributed each year.
Which decision would depend on which ancestry had a majority/minority in a certain community?
It is a peculiarity of the question recommendation system that, while writing a question, other comparable questions come up than those listed after the question is actually published. So only now I find that What exactly is "National Origin” with respect to US anti-discrimination laws? gives a possible answer to my question.
The term "national origin" comes up in Title VII of the Civil Rights Act of 1964 as a prohibition to discriminate against by employers. The term seems to be used synonymously to "ancestry". Answers to the question point out that the term is lacking a good legal definition, but I can see that if someone self-identifies as having a certain ancestry and finds out that economic or other statistical data about others that do the same are less favourable than the national average, this would be a starting point for a discussion about discrimination.