UI Background is the background on the Halo Custom Edition menu.
UI Background [User Interface Background] is the animation in the background of the Halo Custom Edition main menu.
Changing The UI BackgroundEdit
The UI Background file can be found in the "Maps" folder, of the Halo Custom Edition folder. The file named "ui.map" is the UI Background animation; after having another "ui.map" to replace your current UI Background, just over-write the old.
Once Halo Custom Edition is executed, the new "UI Background" should take effect.