These are services that you don’t need to use directly because the functionality that they provide is exposed through the Module methods (described in Chapter 18) or used by AngularJS behind the scenes. I have described these chapters because knowing how they work helps you understand more about AngularJS and can be useful in unit testing.
Source: Freeman Adam (2014), Pro AngularJS (Expert’s Voice in Web Development), Apress; 1st ed. edition.