The actual residence of a person is referred to as their "home address". Since you refer to taxes, this is what the IRS Form 1040 (the personal income tax form in the United States) asks for. An address where they can be contacted is referred to as their "mailing address". This can be a Post Office Box (P.O. Box) where mail is held for them until they retrieve it.
However, there is no common term for what you are describing—at least in the United States, the government does not restrict where someone is entitled to live (except in the form of required property taxes and the like). For senior government officials, especially diplomats, however, there is the term "official residence", but this generally does not apply outside of government.
If you rent a flat from the government
(does it happen in your country?) how
do you call the right to use this
flat?
At least in the United States, property is owned by legal persons, which can be an individual or a corporation; i.e. people own the houses, apartments (flats), office buildings, etc. themselves and they may rent them out to others. The term "tenant" or "lessee" describes someone having paid the rent; the noun "lease" describes the agreement between the two parties; the term "lessor" describes the person who owns the property and is renting it out.
Although the government does own some of the buildings it operates offices in, it also rents other offices—in this respect, the government is not supreme—it is not the supreme owner of all the land. (Although in a few unusual circumstances, it may rent out some living/building space, but this is by no means the norm, and it would be the same as renting from any other person, so there is no special term). Legally speaking, there is the power of eminent domain, enacted by the Fifth Amendment to the Constitution, but it requires that people be paid if the government seizes the land, which it only does so in exceptional circumstances (e.g. public safety).
N.B.: Note that while I generally use the verb "rent" in this answer so as not to confuse the verb "lease" and the noun "lease", the verb "lease" means the same thing as "rent".
