When you import assets, Final Cut Pro uses the imported alpha channels to composite image layers over one another. You can import graphic files that use different methods of handling alpha channels, such as graphics that contain straight or premultiplied alpha channels. Or you can use one image to impose an alpha channel onto another.
Depending on the type of alpha channel used by the assets in your project, the end results may differ slightly. In most cases, graphics that have a premultiplied alpha layer will yield the best results. For more information on alpha channels and compositing in Final Cut Pro, please see the Final Cut Pro Help guide (choose Help > Final Cut Pro Help).