Vision can be set to use either the project IdP or the project user source for authentication.
Perspective requires the IdP be used.
We probably could/should hide the user source setting if you don't have Vision installed, but there's an argument that settings being sometimes invisible is also confusing.
Separate from the project properties, Ignition's internal IdP implementation delegates to a user source, because user sources were around for much longer than Perspective and IdP integration in Ignition, so lots of customers already had user sources set up. However, there's no requirement that you use Ignition's identity provider; you can also hook up a third party IdP which has its own repository of users and authorizations.