What does a flag with an "R" indicate?

A flag with an "R" indicates "Minimum Reception Altitude." This altitude is significant in the context of instrument navigation and the operation of navigational aids. Specifically, the minimum reception altitude is the lowest altitude at which a pilot can expect reliable reception of a specific navigational signal from a ground station. This is crucial for ensuring that the aircraft can properly receive and utilize navigational information, especially in areas where terrain may obstruct signals or where the coverage of the navigation aid is limited.

Understanding minimum reception altitude helps pilots avoid operating at levels where they might not be able to receive essential navigational signals, thus ensuring safety and accuracy during flight operations, particularly in instrument flying conditions.