no good naming convention for the provide/useClass divide in Angularland module providers

Does FooService use FooContract or IFooService (even if IFooService is not really an interface)? Evidence of no plan:

• https://angular.io/guide/dependency-injection-providers
• https://offering.solutions/blog/articles/2018/08/17/using-useclass-usefactory-usevalue-useexisting-with-treeshakable-providers-in-angular/
• https://medium.com/@michelestieven/angular-writing-configurable-modules-69e6ea23ea42