AWS has the notion of "partitions", which is a technical boundary encompassing multiple regions. This mostly doesn't come up, but it does poke through in certain implementation details, like how AMI manifests for groups of regions (partitions) need to be encrypted for different public keys. Each partition has a specific region which must be targeted for certain partition-wide actions, such as managing IAM endpoints in other regions.
Normal AWS (`aws`) traces to `us-east-1`. AWS GovCloud (US) (`aws-us-gov`) is distinct, based in `us-gov-west-1`. AWS in China (`aws-cn`) is distinct again, based in `cn-north-1`.
The AWS European Sovereign Cloud is implemented as a distinct partition – `aws-eusc` based in `eusc-de-east-1` – so it has exactly as much in common with normal AWS as AWS GovCloud (US) or AWS in China.