A display flag must be a digit between 0 and 9, where 0 is the normal mode, 1 is standout mode, 2 is inverse video, 3 is bold, 4 is dim, 5 is blink, 6 is invisible, 7 is underline, 8 is protected, and 9 is alternate character set. This error is produced by the vuform command while validating the display flags for a particular field.