Edit Button...

Edit Button lets you edit the action for the selected button and specify ButtonBox options.

The Button Style pop-up menu lets you select standard button functions that are defined in the style sheet associated with the current notebook.

The other items in the dialog box can be used to set individual options associated with the button.

See Reference Guide: ButtonBox, ButtonData, ButtonFunction, ButtonEvaluator, ButtonNote, ButtonSource, ButtonStyle.

See The Mathematica Book: Section 2.11.6.