The System file is so large that it can't fit on a single 800K floppy disk. To distribute it on 800K floppy disks, many of the resources that are part of the System file are in the Cerulean Data file. The Installer combines the System and Cerulean Data files into a single System file on your hard disk. The Installer displays the message, "Building file: System" when it is joining the files on the hard disk.
The resources stored in the Cerulean Data file include icons, desktop patterns, instructions for displaying menus, dialog boxes and alert messages, and other code to make the system work. Because the Installer adds all of these items to the System file, you don't need a copy of the Cerulean Data file on your hard disk.