What does an underlined frequency in a communication box indicate?

An underlined frequency in a communication box is utilized to signify that there is no voice transmission occurring on that particular frequency. This formatting choice helps air traffic controllers quickly identify frequencies that are non-operational for voice communications, which can be critical in ensuring that pilots and controllers are aware of where to direct their attention.

Understanding that an underlined frequency indicates the absence of voice communication aids in maintaining situational awareness, allowing pilots and controllers to avoid potential confusion and focus on operational frequencies. It is important for pilots and air traffic controllers to be able to distinguish between various types of frequencies and their statuses to facilitate effective communication and ensure safety in the airspace.