Assume your joystick has only 6 buttons, but you want to assign 10 different key inputs to your joystick buttons. What can you do?
Then, you can configure extra key assignments for a virtual joystick number and you can switch the key assignments by pressing a special joystick button (a.k.a. SHIFT-key button).
For example, by default you're using key/mouse assignments defined for Joystick #1. But in the example below, you can switch to key/mouse assignments defined for Joystick #3 whenever "button 3" is pressed.