When registering a .US domain, you will need to fill out additional requirements before purchasing. Starting with the Nexus Code, you will need to select the code which is most relevant to the usage of the domain.
Nexus Category 1
C11: A natural person who is a United States Citizen.
C12: A natural person who is a permanent resident of the United States of America, or any of its possessions or territories.
Nexus Category 2
C21: A U.S.-based organization or company formed within one of the fifty (50) U.S. states, the District of Columbia, or any of the United States possessions or territories, or organized or otherwise constituted under the laws of a state of the United States of America, the District of Columbia or any of its possessions or territories or a U.S. federal, state, or local government entity or a political subdivision thereof.
Nexus Category 3
C31: A foreign entity or organization that has a bona fide presence in the United States of America or any of its possessions or territories who regularly engages in lawful activities, sales of goods or services or other business, commercial or non-commercial, including not-for-profit relations in the United States.
C32: A foreign entity that has an office or other facility in the United States.
The Nexus Codes referenced above can be found here
More information about requirements and policies can be found here