The Fn key lets the user switch the mode of operation of the function keys at any time. The user selects the default mode of the function keys by means of the Function-keys checkbox in the Keyboard Control Panel.
The Function-keys checkbox lets the user choose whether the function key operations are primary or secondary. "Function keys primary" means the function keys are normally in their F1–F12 mode of operation and pressing the Fn key selects their control-button mode. "Function keys secondary" means the function keys are normally in their control-button mode and pressing the Fn key selects their function-key mode.
In other words, pressing the Fn key reverses the mode of operation of the function keys from the default mode set by the checkbox. Table 3-7 summarizes the checkbox settings and the operation of the Fn key. The operations of the individual function keys are shown in Table 3-8 and Figure 3-8.