The United States Federal Aviation Administration (FAA) has a system for categorizing public-use airports (along with heliports and other aviation bases) that is primarily based on the level of commercial passenger traffic through each facility.
To qualify for the AIP, they must have at least 10 aircraft based there but handle fewer than 2,500 scheduled passengers each year.
This means that most aircraft are small and are operated by individuals or other private entities, and little or no commercial airline traffic occurs.
Most of the remaining airfields that qualify for funding are commercial service airports and are more dependent on regularly scheduled commercial airline traffic.
Primary airports are further subcategorized based on the number of passenger boardings as a fraction of the national total.