In this dialog, you can see an excerpt from the Design
If you insert a new control
here, you can define individual styles to be used in the current application.
The user-defined styles are saved as layout files in the portal directory
internal\application\store\GUID of the application\layout
By right-clicking on a control, you can add a new control element by using the context menu.
Here, you have the option to create a completely empty control element or to adopt the styles of basic controls.
When you add a new control in the application designer, make sure that you use a title that is unique across the entire portal. A title may not appear in another application of the current portal.
All information on this topic can be found here