HP XP7 Storage - Manual (Page 284)

Description Item
Select the check box, then Enable or Disable when using or not using Smart Tiers
or Smart Tiers Z. Smart Pool
A pool that consists of the external volumes to which the cache mode is set to
Disable cannot be changed from Disable to Enable.
In Fast Snap, you cannot change the setting of this function.
284 Thin Provisioning Z and Smart Tiers Z GUI reference