The way OpenScatter works is by allowing you to choose an emitter object as the surface. When you've selected a surface you can add scatter system to it. All scatter systems will be using the emitter object that you have selected when you add a new scatter system. You can also choose custom surfaces for each individual scatter system. This can be useful in some instances, but it often makes more sense to use the emitter object.

Using the emitter object, allows OpenScatter to keep track of all the scatter systems and where they belong. For example, you can have one surface where you have some scatter systems. Now if you choose another surface as the emitter object, the other scatter systems will still remain, but you'll be able to create and edit scatter sytems on the new emitter object. This way you can easily switch between different emitter objects.

If you need it, you can also choose to use multiple surfaces. You do this by creating a collection containing your surfaces and then selecting that collection in the addon panel.