The 5U NAS will likely be network attached Storage server. Basically, one can load as many drives as possible (20 in this case), and combined the storage space into one massive drive. There are safety measures to take in place to assure that data loss is minimal. In this instance, the 20 drives will only have 18 used for storage calculation. Therefore, if each drive is an 12TB drive, then the server will show itself as a 216TB drive. The 5U server can be essential for either a media server, or a NAS server.
A media server will host all of the media that the premise will have. This will typically include movies, and TV shows, but music and pictures will also be options. For organization, something like Movies/year/Title might be an option. For TV Shows, that could be TV/Show/season/episode. I am sure you get the idea. Obviously, one does not have to load up the server with all of the drives. They could add as they are getting close to being filled, and using Kal-OS, you will be able to use different size drives.
A NAS (Network Attached Storage) server is intended to provide a resource for hosting less used but essential files, as well as act like a backup. Many experts recommend having a backup system that is 2 times the capacity of the drive. This means a 128GB storage should have 256GB of backup storage. It is not recommended to plug in external drives to the server. This is because if the external hard drive fails, then you lose that content. If a single drive fails in a NAS server, then the system has RAID of your backup in place. It will mark that drive as dead, and send a message to the owner that the particular drive is dead. This will give time for the owner to buy a new drive, and allow the server to rebuild the drive’s content.