This is the exit button of every window. By clicking on this button, it will have the same effect as the close button of the operating system windows at their top right corner, which means that it will simply close the window where it is clicked on. Depending on the window, some action could be completed or done before closing, as for example in the text editor, where the software could ask the user if he/she wishes to save the document before it is closed, or even in the ping windows, where the ping is stopped before the windows are closed, among others. For example, on the mass ping windows, the software will take some more time to close the window if it has still some pings in progress, because it will first close all the threads used by those pings in that mass ping operation, and only after that it will close the window, which is something that happens on other windows also. Basically, a window will only be closed when what it was doing is terminated first, being a quick and safe way to stop any operation in progress in any window before closing it. This button, when clicked on the main window, will only close the main window and not the others opened through it, because when the others are created, those "child" windows are already created independently from their "mother" window and can operate alone.