From the Document: "A 'No-Fly Zone' (NFZ) is a military operation to ensure that no aircraft operate in the airspace of a defined geographic area. An NFZ could also suppress and/or destroy air defenses in a defined region. Many defense analysts consider maintaining NFZs to be combat operations, likely involving actively shooting down and/or destroying a hostile country's aircraft, therefore requiring rules of engagement detailing how and when forces can engage a violating aircraft. NFZs are generally international operations, and U.S. participation in them entails oversight and funding considerations by Congress. The United States has participated in four declared NFZ operations[.]"
CRS In Focus, IF12059
Congressional Research Service: https://crsreports.congress.gov/