This is the value I used to define a CommonButton for the dialogue.
You can have multiple buttons in your dialogues to be able to influence the progression of your workflow.
Here is how this can be used. 
import clr
clr.AddReference("RevitAPIUI")
from Autodesk.Revit.UI import *
dataEnteringNode = IN[0]
buttonlist = TaskDialogCommonButtons.None|TaskDialogCommonButtons.Ok|TaskDialogCommonButtons.Yes|TaskDialogCommonButtons.No|TaskDialogCommonButtons.Cancel|TaskDialogCommonButtons.Retry|TaskDialogCommonButtons.Close
result=TaskDialog.Show("Title",dataEnteringNode,buttonlist)
OUT = result
