What classes should not be added to declarations in Angular?

[akhils-nair-b6a2a78b (Akhil S Nair)]

The below class types should not be added to declarations:

1. A class which is already declared in any another module.
2. Directives imported from another module.
3. Module classes.
4. Service classes.
5. Non-Angular classes and objects, such as strings, numbers, functions, entity models, configurations, business logic, and helper classes.