@SpencerJ and the Santa elves have been working in the background
We have a single "unRAID" array(*) and multiple user-defined "cache pools", or simply "pools". Data devices in the unRAID array can be formatted with xfs, btrfs, or reiserfs file system.
A pool can consist of a single slot, in which case you can select xfs or btrfs as the file system. Multi-slot pools can only be btrfs. What's unique about btrfs is that you can have a "raid-1" with an odd number of devices.
With 6.12 release: