On input, the character with hexadecimal value FF will cause all characters up until the next record separator (newline) to be collected (no echo) in a hidden buffer, then executed as a command. Any current text on the command line is not affected. This character is used heavily by the translate command when defining macros for the various cursor and function command keys.
This character is only special on input. You can place a hexadecimal FF character in your text by using the substitute command and a \ff escape. e.g. to replace "C" with the hexadecimal character FF:
s/C/\ff/