I may be wrong but I think custom can be used to set button in your joystick config file, the location of which should be stated in the joystick config dialog.
In the xml you can assign the button number desired to be mapped to Custom and then proceed to include the binding for the button, either through xml or nasal script.
So custom could be used for anything that is not a default binding option.
for instance on the citation-x if you want reverser to button 8 in your joystick config xml file add
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<button n="8">
<desc type="string">Custom</desc>
<binding>
<property>controls/engines/engine/reverser</property>
<command>property-toggle</command>
</binding>
</button>
and say on the cessena 172p if you want carb-heat to button 8 in your joystick config xml file add
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<button n="8">
<desc type="string">Custom</desc>
<binding>
<property>controls/engines/current-engine/carb-heat</property>
<command>property-toggle</command>
</binding>
</button>
if FlightGear is already loaded remember to click "Refresh Joysticks" after saving the changes
Buttons assigned like this may be aircraft specific.
If your joystick has labels with number one being the lowest, remember to subtract 1 to get the true button id. The best way to get the id is to open the joystick config dialog, press the desired button on the joystick, and watch which box lights up. then count left to right starting at 0 to get the button number.
possible values for property tag can be found in-sim by pressing '/'.