The Mojo Embedder API is an unstable, internal API to the Mojo system implementation. It should be used by code running on top of the system-level APIs to set up the Mojo environment (instead of directly instantiating things from src/mojo/edk/system).
Example uses: Service Manager, to set up the Mojo environment for Services; Chromium code, to set up the Mojo IPC system for use between processes. Note that most code should use the Mojo Public API (under src/mojo/public) instead. The Embedder API should only be used to initialize the environment, set up the initial MessagePipe between two processes, etc.