What is the zipper line on an airport diagram used to indicate?

The zipper line on an airport diagram serves to indicate the boundary between taxiway frequencies. This line is crucial for pilots and ground personnel as it helps to identify the areas where different communication frequencies apply for taxiing operations. Understanding where these boundaries are ensures that pilots can communicate effectively with ground control and follow the correct procedures while taxiing to and from the runway. This distinction is particularly important at busy airports where multiple frequencies may be in use, allowing for better situational awareness and coordination on the airport surface.

The other choices do not accurately reflect the purpose of the zipper line. For example, the end and start of the runway are marked by distinct lines and symbols that indicate physical features of the runway itself rather than communication boundaries. Similarly, while visual approaches have their own markers and symbols, they are not represented by the zipper line; rather, they relate to the approach path and its compliance with visual navigation standards.