You can customize widgets to display a different label or a different set of buttons than the default. For example, you may want to be able to delete an instance from your project by clicking on a button in the InstanceListWidget that was assigned to the responsible_for slot. To display a delete button:
Double-click on the InstanceListWidget labeled Responsible For in the Form Editor.
Click the Buttons tab.
Click the check box in front of Show Delete Instance Button to select it.
Click OK. The InstanceListWidget for the responsible_for slot now has five buttons.