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Regular is for ordinary citizens. Official is for most government officials other than diplomats. Diplomatic is

for diplomats and other US officials on official business or assigned to embassies and consulates.

Official passports are issued to high level government officials who do not have diplomatic immunity. Diplomatic passports have varying levels of immunity from prosecution abroad. Regular passports are just authorization to leave country where you reside; visa is for letting you enter a foreign country. All passports require that.