HP Array Controller HSG V8.7 Software - Manual (Page 119)

Creating Storagesets 3–41
Membership
Indicate to the controller that the RAIDset you are adding is either
complete or reduced, which means it is missing one of its members:
n NOREDUCED (default) indicates to the controller that all of the
disk drives are present for a RAIDset.
n REDUCED lets you add a RAIDset that is missing one of its
members. For example, if you dropped or destroyed a disk drive
while moving a RAIDset, you could still add it to the subsystem by
using this switch.