Storage Replica (SR): Windows Server 2016 introduced a new feature called "Storage Replica". This feature enables block-level replication of an active source volume to a passive destination volume located on another physical server. Source and destination volumes can reside within the same cluster or within separate clusters.

Because Storage Replica operates on the Operating System (OS) layer, it is both location-agnostic and storage-agnostic:
- Location agnostic: this means that volume replication can go to any location where a Windows Server is running, be it another datacenter or IaaS (eg. VM on Azure, AWS, Google Cloud, IBM Cloud, etc).
- Storage agnostic: this means that on one site you can have Hyper-V 2019 on SAN, whereas on the other site you can have Hyper-V 2019 on S2D.

Hyper-V Replica (HR): Hyper-V Replica is an integral part of the Hyper-V role. This feature enables block-level log-based replication of an active source VM to a passive destination VM located on another Hyper-V server or to Microsoft Azure (requires Azure Site Recovery, which is a paid external service, i.e. not part of Windows Server 2019).

Because Hyper-V Replica operates on the hypervisor layer, it is storage agnostic. This means that on one site you can have Hyper-V 2019 on SAN, whereas on the other site you can have Hyper-V 2019 on S2D.