Customization Files under the Files Tab allows you to do the following:
Main Customization File
Specifies the default location of the main customization file (acad.cuix).
The CUIx file is a file that contains multiple XML-based files and custom image files. You modify a customization file through the Customize User Interface (CUI) Editor. CUIx files replace the CUI, MNU, MNS, and MNC files that were used to define menus in releases prior to AutoCAD 2010. ( I miss the MNU files)
Enterprise Customization File
Specifies the location of an enterprise customization file.
The Enterprize CUIx file is typically controlled by a CAD Manager. It is accessed by all users and is stored in a shared network location. The file is set to read-only to prevent the data in the file from being changed by someone that does not know how to manage it. A CAD Manager creates an enterprise CUIx file by modifying a main CUIx file and then saving the file to a shared network drive. Users can then set this name so that they see it.
Custom Icon Location
Specifies the location for custom icons used in customization files. This must be in the search path correctly or you could end up with smiley faces for icons.