Keyboard Codes
The fourth outlet of the key and keyup objects output a keyboard-independent code you can use to detect key presses and releases as described below.
Letters and Numbers
When pressing a letter or number key, the Unicode (ASCII) value of the key is output, ignoring any modifiers. For example, if you hold down the shift key and press the A key, the Unicode value for lower-case 'a' (97) is output.
Other Characters
When pressing a key that is a character key but is not a letter or number, the Unicode value of the key symbol that would be output by the keyboard when the shift key is not depressed is output. For example, on U.S. keyboards, there is a key with both a single and double quote. When this key is pressed, the key object will report single quote. This rule applies to other modifier keys as well.
Function Keys
To the greatest extent possible, keys such as function, home, page up etc. output a consistent values independent of keyboard on each platform. Codes differ between Mac and Windows however.