What does the underscores in _servicename_ mean in AngularJS tests?

In the following example test, the original provider name is APIEndpointProvider, but for injection and service instantiation the convention seems to be it has to be injected with underscores wrapping it. Why is that?

'use strict';

describe('Provider: APIEndpointProvider', function () {

  beforeEach(module('myApp.providers'));

  var APIEndpointProvider;
  beforeEach(inject(function(_APIEndpointProvider_) {
    APIEndpointProvider = _APIEndpointProvider_;
  }));

  it('should do something', function () {
    expect(!!APIEndpointProvider).toBe(true);
  });

});

What is the convention I'm missing a better explanation to?

The underscores are a convenience trick we can use to inject a service under a different name so that we can locally assign a local variable of the same name as the service.

That is, if we couldn't do this, we'd have to use some other name for a service locally:

beforeEach(inject(function(APIEndpointProvider) {
  AEP = APIEndpointProvider; // <-- we can't use the same name!
}));

it('should do something', function () {
  expect(!!AEP).toBe(true);  // <-- this is more confusing
});

The $injector used in testing is able to just remove the underscores to give us the module we want. It doesn't do anything except let us reuse the same name.

Read more in the Angular docs