Before Menu Extras arrived, there was no way in Mac OS X to, for example, show AirPort networking signals, battery levels, and related items. Also, the Control Strip ceased to exist, beginning in Mac OS X. To find a way through these problems, Apple Inc. created a function where these could be shown to the user. All a user had to do was to drag a Dock Extra onto the Dock. They were stored in the Dock Extras folder in the Applications folder on the Mac OS X boot volume. (Just by dragging a Dock Extra on or off the Dock did not alter its position within the folder hierarchies).
Dock Extras can be seen as small, "always-on" mini-applications providing instant-access features from within the Dock. Unlike the icons of other (real) applications, a Dock Extra does not have any interfaces other than a pop-up menu on the Dock.
Dock Extras are one of the few elements that can be positioned anywhere (either in the section reserved for applications or the section reserved for documents, folders and other items), except for to the left of the Finder icon or to the right of the Trash. A Dock Extra is activated by control-clicking the icon or clicking and holding it; its activation is identical to the activation of a contextual menu.
A Dock Extra's icon was constantly variable and in fact, many were designed to be updated on-the-fly.
Dock Extras have the file extension .dock.
Dock Extras disappeared rapidly with the arrival of Mac OS X 10.1 "Puma". (For one, iTunes 1.1.2, bundled with Mac OS X 10.1.0, has lost its Dock Extra; the iTunes controls are integrated with the contextual menu one gets when a user clicks and holds the application icon in the Dock). They are very rare as of "Puma" and, as of Mac OS X 10.2 "Jaguar", have been made pretty obselete. Apple no longer bundles any Dock Extras. Current versions of Mac OS X allow a Dock Extra (if you can find one!) to be dragged onto the Dock, but it has lost all functions (it no longer works in recent releases of Mac OS X.)