Basically, it means you can have RAID 1 and RAID 0 on the same set of hard drives. So say you had 2x500GB drives, and you created a 250GB RAID 1 array, you could then create a 500GB RAID 0 array.
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I.E
Antec P182 / Corsair HX520
Asus P5K Premium/ Q6600
Corsair 2x1GB / 2x Samsung T1 500GB
EVGA 8800GT