When you run the application for the very first time you will be presented with the main window, located in the centre of the screen. This will only happen the first time - subsequent launches will position the window wherever you left it. This is true of all windows - the position of the window is retained betweens sessions.
In addition, whenever you re-launch the application most of the settings from your previous session will be retained, such as the category you were using.
Only one instance of the application may be launched. If you try to run the application, and there is already a version running, then a message will be displayed and the new instance will terminate.