The Fuse file system isn’t the actual file system on each disk, it’s like an overlay that brings together the file systems on each disk. Like a super basic setup assuming you had two data drives is that you could have a share called “MyFile Share” and have two files in the share, “File A” located at /mnt/disk1/MyFileShare/FileA and “File B” located at /mnt/disk2/MyFileShare/FileB but if you connect to the file share remotely or do an ls on /mnt/user/MyFileShare/ you’ll see both files A and B. So you create each share and it’ll distribute all your files across disks according to your specifications.

This is the fuse file system, and it’s how UnRAID implements the “RAID-like” features. Because unlike actual RAID your files aren’t striped across the array, each file lives on one disk. So while you can have 1 or 2 parity drives that can rebuild your array in the case of a lost drive, unlike RAID you don’t lose your entire array. If you have one parity drive and a 5 disk array and lose two data drives, your parity can’t rebuild the lost data but the data that’s on the other 3 disks are still accessible.