Im a dual citizen moving residencies to Romania from Canada. I have accountants on both sides helping me and I have read over the tax legislation, treaties and necessary forms. Its shocking how subjective and ambiguous this whole thing is. They dont even work with border control databases to see when you enter/leave their own country. The canadian gov considers you a non resident as soon as you leave canada to go back to your country of origin (if you are an immigrant like me), whereas the Romanian government considers you a fiscal resident once you fill out a form after 183 days. But the Romanians withold tax at the company level as if you were a resident from day 1. And in the end, none of this shit matters much because in Canada you simply file taxes as a non-resident and the only time when you would need to prove your nonresidency status is if you are unlucky and get audited. Then its this subjective "center of vital interests" questionnaire where they ask you some specific questions to see which country you have stronger ties to. If either country feels like it they can both claim you as a tax resident, then you get Foreign Tax Credits from the country with the higher tax rate (Canada) and you effectively pay the same tax as if you were just a Canadian fiscal resident.