Non-domicile
Non-domicile status is given to people who were either not born here or whose parents spent most of their lives in another country.
A UK 'resident' for tax purposes is anyone, of any nationality, who spends more than half the year (strictly 183 days) in the UK for any reason - work is a common one.
By itself that would normally make them liable for UK income tax on their worldwide income. The anomaly, however, is that the same individual could simultaneously be 'non-UK domiciled' because their 'permanent legal home', as the tax rules put it, is not the UK.
Non-domicile status is given to people who were either not born here or whose parents spent most of their lives in another country.