vSAN Stretched Cluster and vSAN Remote Office/Branch Office (ROBO) are both configurations of vSAN, but they are designed for different use cases.
A vSAN Stretched Cluster is a configuration that allows you to set up a vSAN cluster across multiple geographic locations, typically in the same metropolitan area, to provide additional data protection and availability. This is useful for organizations that need to ensure that their data is available even in the event of a disaster or site outage.
On the other hand, vSAN ROBO is a configuration that allows you to set up a vSAN cluster in a remote office or branch office, typically in a different geographic location from the main office. This is useful for organizations that have multiple remote offices or branches and need to provide local storage and compute resources for those locations.
Both configurations use vSAN to provide storage and compute resources, but the key difference is the geographic distance between the sites in a stretched cluster is generally smaller and the main goal is to provide high availability, while vSAN ROBO is designed for remote locations where the main goal is to provide cost-effective storage and compute resources with the ability to replicate data back to the main location.
In summary, vSAN Stretched Cluster is designed for high availability within the same metropolitan area and vSAN ROBO is designed for cost-effective storage and compute resources in remote locations.