Which type of airspace is generally controlled but not designated as A, B, C, or D?

Class E airspace is commonly recognized as controlled airspace that is not designated as Class A, B, C, or D. It typically exists in areas where there are no specific requirements for air traffic control services, especially at lower altitudes, but it still applies various rules regarding visibility and cloud clearance. Class E is primarily utilized for instrument flight rule (IFR) operations and can extend from the surface to various altitudes.

Airspace classified as Class A, B, C, and D has specific operational requirements and different levels of service from air traffic control, whereas Class E encompasses the remaining spectrum of controlled airspace not included in those categories. This makes it versatile for a range of flying activities, including general aviation, while still maintaining some level of regulatory oversight to ensure safety.