The term originates from the Allied Signals Book (ATP 1), which in the aggregate is for official use only. Signals are sent as letters and/or numbers, which have meanings by themselves sometimes or in certain combinations. A single table in ATP 1 is called "governing groups," that is, the entire signal that follows the governing group is to be performed according to the "governor." The letter "B" indicates this table, and the second letter (A through Z) gives more specific information. For example, "BA" might mean "You have permission to . . . (do whatever the rest of the flashing light, flag hoist or radio transmission says) "BZ" happens to be the last item in the governing groups table. It means "well done".