BaseElements ships as two optional file types, .fmp12 files and a runtime application. But in both situations if you open the BaseElements files with FileMaker Pro or Pro Advanced, you have full access to modify Layouts and add your own Scripts to the BaseElements_UI file.
In order to do this with the runtime files, you will first need to open the main BaseElements.DDR file with FileMaker Pro.
When you modify layouts, you can add your own versions of layouts, or duplicate layouts and change the formats. If you want to, you can add your own Fields, Buttons or Tabs to any layout.
As well, the BaseElements Data file is not locked for access by other tools. So you can also create separate layouts in another file that have Table Occurrences referring to Base Tables in the BE Data file. This allows you to have separate User Interfaces for your BaseElements data.
BaseElements main file doesn't allow editing.
BaseElements_UI allows you to edit existing Layouts, add new Layouts, and add new Scripts.
BaseElements_Data allows you to add new Layouts, add new Scripts and connect another file to the base Tables.