For each Data ONTAP system that you want to use array LUNs, you must assign an array LUN to it and create its root volume before you can install Data ONTAP.
At this point it is easiest if you assign only one array LUN to each Data ONTAP system that you want to use array LUNs. You can add additional array LUNs after you install Data ONTAP and verify your configuration.
For Data ONTAP 8.x, the root volume must be a FlexVol volume. You should assign only one array LUN to the aggregate with the root volume. The FlexVol volume can then use all of the space for the root volume.