Use Map Key to change the behavior of a key.
Adding or Finding a Key
Click the + sign to add or find a key that you plan to map. If the key is already in the list of explicitly mapped keys, it is highlighted in the Key column when you exit the dialog box. If the key is not in the list of explicitly mapped keys, it is added to the list when you exit the Add or Find Key dialog box.
Explicitly mapped keys
The keys in the list of Explicitly mapped keys include those that do not send characters (such as function or control keys). The Key and the X Keysym entry in the left panel is a short summary of the mappings; select a Key to see complete mapping information in the right panel.
Although you can choose to add any key to this list, it's likely that you can perform all necessary mapping using the keys listed here. Other keys (in particular, those that send characters) are implicitly mapped. Review these on the Characters tab.
Key... Maps to these Keysym(s)
The current behavior for an entry in the list of Explicitly mapped keys is displayed. To change it, use the Change Mapped X Keysym options in the right panel (Unshifted, Shifted, Alt Gr, Shifted Alt Gr). In general, only one X Keysym (the one in the unshifted position) needs to be defined for any given key.
Require fixed keycode
You can also specify a specific keycode, although this is generally not necessary. Click Require fixed keycode and type in a value between 8 and 255.The keycode is a number that identifies a row in the keymap. The definition for the selected key will be placed in the specified row of the keymap.