Stretched Clusters provide the ability to protect an SDDC running in VMware Cloud on AWS from an Availability Zone failure. If you would like to know more about the Stretched Clusters capability, sometimes referred to as Multi AZ SDDCs in VMW on AWS, make sure you read this article by Emad Younis, this post by Frank Denneman and this article by Glenn Sizemore. In addition, as announced in December 2019, there is a 95% discount on the cross-AZ traffic between the AZs.
Just like a standard vSAN cluster, storage consumption in a Stretched Cluster is managed using storage policies.