Most high-income countries tax worldwide income: once you're a tax resident, your salary, foreign clients, overseas rental income, and investment gains are all potentially in scope, wherever they arise. The United States goes furthest, taxing its citizens on worldwide income even when they live abroad.
Worldwide systems usually offer relief from double taxation through foreign tax credits or exclusions, and through tax treaties. But the default is broad: moving to a worldwide-tax country generally means that country wants a share of everything you earn globally.
Why it matters for your move
If you earn meaningfully from foreign sources, moving into a worldwide-tax country can quietly raise your total tax bill even if local rates look reasonable. It's the flip side of the territorial advantage.