A snap-in is the basic component of a console. You can use snap-ins only within consoles; you cannot run them independently of a console.
When you install a component that has a snap-in associated with it, the snap-in is available to anyone creating a console on that computer (unless restricted by a user policy).
Snap-ins and snap-in extensions
MMC supports two types of snap-ins: stand-alone snap-ins and snap-in extensions. You can add a stand-alone snap-in, typically called a snap-in, to a console without adding another item first. A snap-in extension, typically called an extension, is always added to a snap-in or to another snap-in extension that is already in the tree. When extensions are enabled for a snap-in, they operate on the objects controlled by the snap-in, such as a computer, printer, modem, or other device.
When you add a snap-in or extension to a console, it might appear as a new item in the tree, or it might add shortcut menu items, additional toolbars, additional property pages, or wizards to a snap-in already installed in the console.
Adding snap-ins to a console
You can add a single snap-in or multiple snap-ins and other items to a console. In addition, you can add multiple instances of a particular snap-in to the same console in order to administer different computers or to repair a damaged console. Each time you add a new instance of a snap-in to a console, any variables for the snap-in are set at default values until you configure the snap-in. For example, if you configure a particular snap-in to manage a remote computer, and then you add a second instance of the snap-in, the second instance will not automatically be configured to manage the remote computer.
Typically, you can only add snap-ins that are installed on the computer you are using to author a console. However, if your computer is part of a domain, you can use MMC to download any snap-ins that are not locally installed but that are available in Active Directory. For more information about adding published snap-ins and extensions to a console, see Add, Remove, and Organize Snap-ins and Extensions in MMC 3.0.