custom-reset-menu

custom-reset-menu is a variable defined in `cus-edit.el'.
Its value is

(("Undo Edits in Customization Buffer" . Custom-reset-current)
 ("Revert This Session's Customizations" . Custom-reset-saved)
 ("Erase Customizations" . Custom-reset-standard))

Documentation:
Alist of actions for the `Reset' button.
The key is a string containing the name of the action, the value is a
Lisp function taking the widget as an element which will be called
when the action is chosen.