- #include <disclaimer.h>
There are in fact two types of such menu bar items, one known as NSMenuExtra's and another known as NSStatusItem's. The former are loaded and displayed by SystemUIServer, a process provided by the system. The latter can be displayed by any app written by any developer. One good thing about the former is that you can rearrange them by ⌘-dragging the menu items.
Anyway, due to this better behavior of NSMenuExtra's, people often wanted to write their own. In fact until and including OS X 10.1, Apple allowed it. But since 10.2, Apple had a code that blocked SystemUIServer to load non-system-provided NSMenuExtra's. But until Yosemite, there was a known way to work around it, available as an open-source code as MenuCracker. WanNianLi used this to inject their own NSMenuExtra's to SystemUIServer.
Essentially, until Yosemite, SystemUIServer had a fixed list of allowed NSMenuExtras. MenuCracker was an NSMenuExtra that pretended to be one of those allowed ones, which, once loaded inside SystemUIServer, removed these checks, so that more NSMenuExtras can be loaded without any problem. In OSX 10.11 El Capitan, Apple added a more stringent check of the allowed NSMenuExtra's, and MenuCracker no longer works.