In MacOS X, the feature this flag enables is called click-through. It means that clicking on the window passes the click through to the control the mouse was under at the time even when the window does not have the focus. As a rule, this behavior should not be used, but there are times when it is quite useful, such as for system monitor programs, the Deskbar, and so forth. Typically, this flag is used for "helper" applications and utility windows.