灶君: Possibly this (from wikipedia )
But original 7 segments displays – YOU chose how to light each segment…
so the 0 – 9 and other combinations were sent to your decode circuit… and you chose how to display data.. even as far as going to all letters of the alphabet..
If the display has a 4-bit binary input rather than a pin for each segment, It depends on how the display is programmed. Some of them might display hexadecimal characters.
The ancient 744511, for example, blanks the display when values greater than 9dec (1001bin) are input.
It depends on the design of the specific controller;
some will display hexadecimal digits (A-F)
some will blank the display
some will show other symbols (for example '-')
others will show random patterns
Read the datasheet for the specific display and driver you're using.