Countries generally have rules around residency that boil down to "if you spend most of a year within the country you are resident in that country".

Tax residency is separate and the rules are a bit different and again vary by country.

For the UK, basic rules that cover residency for most are listed at <https://www.gov.uk/tax-foreign-income/residence>, with a link to the full rules for those not covered by the basic rules.

> what does it mean “residency”?

Usually residency is self declared or based on where you have spent the majority of the year working in.

This is already handled, by lots of people, when they declare residency already, for where they currently live/work/are covered by taxes under.

This isn't a new thing.

> In the UK there’s no address registry where you declare your address

Can you vote in Glasgow's local elections if you live in Manchester?

It would be more accurate to say there is no single register of addresses.

The electoral register knows I’ve left the U.K.; I had to tell HMRC separately because they don’t share info; likewise the Student Loans Company even though there is a close connection between income and repayments [0]; and when I tried to tell the DVLA, they told me they lacked the capacity to know about non-UK addresses.

[0] the repayment rules means that, for low- and mid-income people, it behaves more like a graduate tax on worldwide income than a loan — it’s a percentage of pay over a threshold, and only high earners can pay it off fully — and if you do a self-assessment tax return there is a box for student loan stuff.

To vote, you need to register to vote, I voted at London Mayor elections in 2016. You vote wherever you register to vote, I don't remember the details but I guess it was me declaring that I live there and I would like to vote.

In UK there's this concept called proof of address and it's usually bill or a document that is sent to your address, there's no central registry of residents.

The point is, you don't need a register, central or otherwise, for the place of residence to be legally relevant. The way this kind of things work is that, should there be a legal challenge, you will provide a proof of residence to the court or whatever relevant body, just like you do when registering to vote.

There are clear rules regulating that, but they may depend on the country.

The rules are anything but clear.

Some countries require at least 6 months + 1 day to be considered a resident, others might have you have a resident even if you aren't there if "the country is the center of your family or economic interests".

The rules for the UK are set out pretty clearly in the statutory residence test [1]. It covers all such cases in more detail than you ever wanted to know :)

What causes difficulties though is that the rules in different countries, even if clearly defined, can contradict each other on the question of tax residency. So you may well be liable to pay taxes in two (or more) countries at the same time.

You may even have to pay taxes in both countries on the same income or gains, unless there is a double taxation agreement that allows you to offset some of those taxes against each other [2]

[1] https://www.gov.uk/hmrc-internal-manuals/residence-domicile-...

[2] https://www.gov.uk/government/collections/tax-treaties

They may also be very unclear or outright missing, depending on the country.

It will depend on the law.

Tax law favours a designated location for residency, for example.

The GDPR doesn't even operate on the level of residency (at least for data subjects). You're covered by the GDPR when you're physically in an EU member state[0] as regards your activity in that member state or if the data controller/processor is established in the EU. The UK GDPR will be the same, mutatis mutandis.

[0] Or somewhere where EU law applies by virtue of international law, like an embassy, an EU-flagged ship, an area of Antarctica claimed by an EU member state etc. etc.

If you get a Turkish residence or work etc visa and make that your primary residence then that's your residence.

If you're there on your 90day tourist visa, it's not your residence.

> How FB would know about it? In a matter of fact GDPR applies to people in EU, and data processed in EU.

It also states that data from people in EU has to be processed and stored in the EU.

It's not a matter of citizenship nor residency. Facebook just geo-locate you IP endpoint (so endpoint of your VPN) and manage your data by doing so. So USA rules will apply.

BUT if you are browsing websites hosted in Europe, EU rules will apply to your data.