Custom Widget
Setting a master to behave as a custom widget causes it to behave like a built-in widget when it is added to a wireframe. The widgets in the master are added to the wireframe with no connection to the master so the properties, annotations, and interactions can be changed on any of the widgets. This is useful to create your own collection of commonly used widgets with predefined properties such as a button shape set to the tab shape with a grey fill color.
You can change the default behavior of a master to be a custom widget by right-clicking a master in the Masters pane and selecting Behavior->Custom Widget.