"Shared VPCs" are a unique AWS infrastructure design: https://docs.aws.amazon.com/vpc/latest/userguide/vpc-sharing.html

See prior work/explanations/etc here: https://issues.redhat.com/browse/SDE-1239

Summary is that in a Shared VPC environment, a VPC is created in Account A and shared to Account B. The owner of Account B wants to create a ROSA cluster, however Account B does not have permissions to create a private hosted zone in the Shared VPC. So they have to ask Account A to create the private hosted zone and link it to the Shared VPC. OpenShift then needs to be able to accept the ID of that private hosted zone for usage instead of creating the private hosted zone itself.

QE should have some environments or testing scripts available to test the Shared VPC scenario