I'll chose instance in this case. And now I've got that material stored in my scene. Even though it's not assigned to an object. And I can even delete it from the view here. Select it and press Delete. And if I ever need to get that back, I can drag it from the sample slot onto the view, and either instance it or copy it. Sample slots are very useful for storing materials that are not assigned to any objects in the scene, but because there are only 24 of them, you may run out of them pretty quickly. In the following movie, we'll see how to create material libraries to get around that limitation.
