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))

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.