CLASSIC will put the calculator into Classic mode as opposed to MathPrint mode. The Classic mode will make the calculator display everything as pre-MathPrint OS would, including input. For instance, rather than superscripting exponents as MathPrint mode would, Classic mode uses the simple caret syntax (^).
When in Classic mode, text and numbers are displayed much faster on the home screen and the function menus load faster. This can be useful in games that use the home screen, or just with calculations in general.