I do not think this is required, but I am not certain. After installing Commerce 9 with the Habitat and Adventure Works catalogs, it creates four rest environments: Auth, Ops, Minions and Shops. Each environment only differs by the wwwroot\config.json field Environment. Respectively for me they are: HabitatAuthoring, HabitatMinions, AdventureWorksOpsApi, HabitatShops. 

The OpsApi did not seem to be working. I do not find an environment in SQL in the CommerceEntities table for an AdventureWorksOpsApi or HabitatOpsApi. I changed the Ops environment to use the HabitatMinions environment and it seems to be working.

Everything seems to be working, so I do not think this is required; but it almost seems to me that there should be four sites per environment. Should every environment have its own set of Auth, Ops, Minions and Shops supporting sites?

And should there be OpsApi environments in the CommerceEntities table?

Update to add to the above. It appears the install of the demo commerce environments uses the AdventureWorksOpsApi for the Ops host configuration even though it does not create a matching environment. The Ops host will not working until you either create one or your change the configuration to one of the others. I chose to change to one of the HabitatAuthoring for now.

From the posts below, I now have a better understand of the role of each host and the purpose the environment in the configuration.