The Index Update physical operator updates rows from its input in the nonclustered index specified in the Argument column.
If a WHERE:() predicate is present, only those rows that satisfy this predicate are updated. If a SET:() predicate is present, it indicates the value to which each updated column is set. If a DEFINE:() predicate is present, it lists the values that this operator defines. These values may be referenced in the SET clause, or elsewhere within this operator and elsewhere within this query.