- An NgModule bounds declarable classes only. Declarables are the only classes that matter to the Angular compiler.
- Nstead of defining all member classes in one giant file, you list the module's classes in the @NgModule.declarations list.
- An NgModule can only export the declarable classes it owns or imports from other modules. It doesn't declare or export any other kind of class.
- NgModule can extend the entire application with services by adding providers to the @NgModule.providers list.