Original: Whether an array runs as RAID 0 +1 or RAID 1 +0 in practice is often determined by the evolution of the storage system. A RAID controller might support upgrading a RAID 1 array to a RAID 1 +0 array on the fly, but require a lengthy offline rebuild to upgrade from RAID 1 to RAID 0 +1.
Simplified: It uses two or three disks, and then partitions them equally to form a combination of RAID 0, RAID 1, RAID 5 or RAID 1 +0 levels.