The second image on the homepage of https://unraid.net shows a "random" screenshot of the unraid dashboard and disk setup, but it looks like the drive capacities have been messed with manually for the screenshot and the array setup makes no sense any more. Parity drive is shown as 8TB when there are two 12TB also in the array - parity should be at least as large as the largest disk, right? Also the 2nd disk description says 4TB but the size columns shows 12TB.
The intention was probably to show that you can mix & match different drives but it could at least be a valid setup.