The QoS policy was a feature included in Windows Server 2012 R2 that enabled to set a minimum and a maximum IOPS on a VHD(X). These policies were not centralized and you had to set the QoS policy on each VHD(X) independently.
In Windows Server 2016, Microsoft has improved this feature because the policy can now be stored in the cluster database. You can create a policy based on a minimum / maximum IOPS and/or a bandwidth. This policy can be applied to a VHD(X) (Dedicated Policy Type) or to a set of VHD(X) (Aggregated policy Type).