What are the vertical limits of ATC Assigned Airspace typically defined by?

The vertical limits of ATC Assigned Airspace are typically defined by Flight Level 180 (FL180). This is a crucial benchmark in aviation because it marks the transition from lower altitude airspace, where regulatory requirements and procedures for aircraft separation differ from those used in higher altitude airspace.

In the United States, airspace below FL180 is generally referred to as controlled airspace (Class E and below), and pilots must adhere to specific VFR (Visual Flight Rules) and IFR (Instrument Flight Rules) regulations. Above FL180, the airspace shifts to Class A airspace where all flights must operate under IFR, and the standard separation protocols apply, enhancing overall safety and efficiency in high-density flight environments.

The other options, while relevant to airspace management, do not specifically define the vertical limits of ATC Assigned Airspace. Class B limits, for instance, are variable and based on specific terminal area requirements, and air traffic routes established by the FAA are lateral, not vertical parameters. Geographical terrain height is also situation-dependent and does not offer a definitive limit for ATC Assigned Airspace comprehensively.