What does the "Sfc-4000 AGL" indication refer to in airspace classification?

The indication "Sfc-4000 AGL" refers specifically to the vertical limit of airspace, indicating that the airspace extends from the surface (Sfc) up to 4,000 feet above ground level (AGL). This designation is crucial for pilots as it helps them understand where specific airspace classifications begin and end, which is vital for navigation, adherence to regulations, and maintaining safe distances from other airspace users.

In this context, knowing the vertical limit of airspace allows pilots to ensure compliance with any applicable restrictions or procedures, especially when operating in areas with varying airspace designs such as controlled, restricted, or special use airspace. It also directly informs the pilot about the altitude at which certain operating rules apply. Therefore, this designation plays a key role in flight planning and situational awareness.