@Injectable({
  providedIn: "root" | "any" | "platform"
})
export class MyService {}

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ProvidedIn: root

Every service defined with 'root' will be provided in the root injector and is a singleton for the whole application. Lazy modules will use the instance from root.

You will still using 'root' for you application for most of time.

ProvidedIn: platform

Every service defined with 'platform' will be provided in the platform injector and is a singleton for all applications. Lazy modules will use the instance from platform.

The difference between 'root' and 'platform' is only noticeable when running multiple Angular application in the same window. Both make sure that only one singleton exists even for lazy loaded modules. But when running two applications in the same window, each application has it's own root injector but both share the platform injector.

This means that the best use case for providedIn: 'platform' is for sharing services over application boundaries. E.g. with Angular Elements.

If you are using Micro Front, and your app contains multi small apps and angular element, for all those apps want to share the same service, for example, AuthService, you can use 'platform'.

ProvidedIn: any

Every service defined with 'any' will be provided in every module it is used. That means there might be multiple instances of the same service. That means that every lazy loaded module has it's own instance of the service. All eagerly loaded modules share one instance provided by the root module injector.

In the following example is the service used twice. Once within an eagerly loaded module (provided by root) and once in Lazy Module B (provided by its child injector).

The idea is similar to AngularJS '.factory()', evey times you use .factory injection, it creates a new instance.