When you install a new application, sometimes the Application Installer will ask you if the application should be installed for you alone or for all users on the system. If you select Just For Me, a new application's folder will appear in your Users folder and the app will be installed there because other users cannot see inside of that folder or use its contents. If you select For All Users, the application goes into the Applications folder at the root level of the hard drive because that folder is accessible or readable by all users on the system. When you install a font, you have the option of installing that font into your user account or all users on the system. Place the font in your User Fonts folder and only that user account will be able to use it or even know that it's there. Place the font in the local library folder Fonts folder and all users can use the font. Try to place the font in the system library Fonts folder and the system won't let you. Fonts in this folder are from Apple and must remain unchanged. Preferences are another great example. When an application preference is changed, that default behavior change is set in a plist file. If you are setting that as a user, well then, that change will almost always be set in the user's Preference folder. If you open a program and its behavior is already set to a default preference, that plist file is in the local library folder. To set custom preferences for all users on a system, you have to write those preferences to the root library. This is effective in lab situations where many new users may be created after initial system deployment and they'll all need to read that file. The domain system is a multilayered hierarchy that logically controls the organization of the operating system to clearly and predictably engage resources at multiple stages in the startup process and during system use. So you can have a reliable, extendable, and personal multiuser operating system.