The GS/OS (version 4.0) operating system does not interpret keystrokes; the extended character sets are handled by the firmware of the Apple IIGS itself. The firmware directly understands only the lower 128 ASCII codes. To access symbols assigned above the lower 128 codes requires examination of the ADB information. No extended characters are recognized automatically.
The Command key (Open-Apple) or Option key (Closed-Apple) can be detected via PEEKs of the game port buttons or via the keyboard modifier returned by GetNextEvent. By checking for these keys, then checking the character key, the programmer is able to assign one of the extended characters to the Command-X or Option-X keystroke.