Appliance 5230 disk capacity

Hi all,

in some documents for appliance 5230 they mentioned that the available storage capacity is 4TB,

in same time the disks slots is 12 disks 2 of them for OS and other 2 not used,

now the remaining numbers of slots is 8 disks and 3 TB disks size is supported lets say 8 disks *3 TB = 24 TB !!!! how com 4 TB is only usable ?

thanks