With MuLab you can work on multiple projects at the same time.
This may be useful, for example, when you have different versions of a project and want to merge them into a 'best of' project.
Or when you want to re-use parts of a certain project in another project.
When you open a project via the PROJECT menu you'll get a question "Replace this project - Open Extra Project". Choosing "Open Extra Project" will obviously keep the current project open and open a new extra project window.
Note that having multiple projects open at the same time will use more CPU of course.
Although you can 'mute' a project via right-click on the play knob -> "Toggle Process On/Off", or via the PROJECT menu.
When you have multiple projects open in parallel, you can copy-paste and drag-drop parts and modules from one project to another.