An AZ is *not* a Region — each Region is composed of several (from 2 in `us-west-1`, up to 5, in some AP regions) AZs.

So, stating that Atlas allows replicas across AZs, and DocumentDB across “just one Region” is a distinction without a difference: I don’t know if that’s a typo (and Atlas indeed allows replication across Regions) but then I’d like to know latency guarantees (AWS Regions are fully independent of each other, so even having a single VPC spanning Regions is not possible)