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

The zipper line on an airport diagram is used to indicate the boundary between taxiway frequencies. This marking helps pilots and ground controllers understand the frequency at which they should communicate while taxiing along certain taxiways. Proper communication is essential for ensuring safe movements on the ground, and knowing where different frequencies apply helps prevent misunderstandings and enhances traffic flow in busy airport environments.

The other options do not correctly reflect the purpose of the zipper line. For example, the end of the runway and the start of the runway are indicated by distinct markings that are specific to runways, while the transition point for visual approaches would involve different elements related to approach paths and guidance rather than ground taxiway frequencies.