This inherited property is not used by the Web Parts control set when rendering a PageCatalogPart control. The property is overridden only for the purpose of preventing it from appearing in Microsoft Visual Studio 2005 designer tools.
Assembly: System.Web (in System.Web.dll)
The property, as a member of the PageCatalogPart control, is not used anywhere within the Web Parts control set for rendering the control. Therefore, you should not use this property; setting it has no effect on rendering.
To prevent developers from using this property, code attributes are set on it to hide it in Visual Studio. When you work with a PageCatalogPart control in Visual Studio, the property is not visible in the Properties pane or IntelliSense.
If you want to use this property, you need to create a custom class that inherits from the CatalogPart class. The custom class can either use the inherited DefaultButton property as it is, or override it and change its behavior.