What does a speed or altitude between two horizontal lines signal?

A speed or altitude displayed between two horizontal lines indicates a mandatory requirement. This scenario is typically found in airspace management, and the two lines represent specific limits that must be adhered to by pilots. When the information is mandatory, it serves as a regulation that pilots must follow to ensure safety and compliance with air traffic control instructions.

Understanding these mandatory requirements is crucial for maintaining safe distances from other aircraft and adhering to specific operational protocols, often related to airspace classifications or flight operations within controlled airspace. Therefore, pilots must be vigilant in observing these limits as they are not simply recommendations, but rather essential rules that govern flight operations in the designated area.