Customization (CUIx) files define elements that make up the user interface and are edited using the Customize User Interface (CUI) Editor.
The program comes with several different CUIx files that can be tailored to your work environment. AutoCAD-based products have two main designations for CUIx files, main and enterprise; by default the main CUIx file is loaded. You can customize the files that come with the program or create your own CUIx files from scratch using the Transfer tab in the Customize User Interface (CUI) Editor.
When customizing the CUIx files, or creating your own, the Customize User Interface (CUI) Editor creates a backup copy of the file when you first start making changes to the CUIx file. This allows you to restore the file if you delete something that you wanted to keep. If you happen to modify an included CUIx file, you can reset it back to its original state.
The Customize User Interface (CUI) Editor allows you to not only create new user interface elements and commands from scratch, but you can also copy existing user interface elements and commands to modify them.