useClass vs useExisting

Question

When should we use useExisting provider instead of useClass?

providers: [
{provide: Class1, useClass: Class1}, 
{provide: Class2, useExisting: Class2}]

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REMARK: I have not found an exact question on SO. And for better indexing decided to create this specific one here, although I found this answers:

• Angular 2 useExisting providers
• useValue vs useFactory
• Angular 2 useExisting providers

but would like to have more real examples

Answer 1

useExisting - create refrence to service example here
useClass - create new instance of service example here