Difference between selection change and modified methods of Form Combo Box control?

When both of this methods would be used while using the combo box in Form Level.I am new to AX.

the super() in selectionChange will calls the modified method. The only difference i can see is that the selectionchange will be triggered only when you select any of the available value from the combo box and the modified will be called irrespective of what value entered in the combo box field.

Thank You So much.