The `ember-engines-router-service` addon provides an API for authoring a Router service within ember-engines, enabling developers to use RouterService APIs like `transitionTo` and `isActive` inside each engine. It also introduces new "external routing" APIs such as `transitionToExternal` and `isActiveExternal` to link external routes together. This project is designed for Ember.js developers working with ember-engines who need advanced routing capabilities within their engines.