Of course, you can configure everything as usual under System Preferences/Displays.

If you have a 2+ display setup for example, you can create a Dummy, mirror it to one display but leave the other display unaffected. Or you can create 2 dummies, mirror each one to a separate display.

Every display can serve as a Main or Extended display, as well as a Mirror for some other display. Therefore a mirrored set can be Main or Extended as well. A typical use case is to use your MacBook display as an Extended display alongside your external display which is a Mirror of a Dummy that is set as Main display (providing all the fine grained HiDPI resolution options).

Hope this helps!