Depends on the country, I think. Usually, if you move to another country, you have to register there so each country knows who is a foreigner. This is also trac

Depends on the country, I think. Usually, if you move to another country, you have to register there so each country knows who is a foreigner. This is also tracked for taxes and social security, and for elections. For elections they need to keep track of each person’s status: EU citizens might be able to vote in local election in another EU country, but not national ones, so they have to record this type of information. These three things are different: you can live in a country, pay taxes in another, and vote in a third. I guess states are just good at keeping records.