what's the relationship between flask-socketio and Engine.IO?

Question

http://flask-socketio.readthedocs.io/en/latest/
there are two description of how the emit use.

flask_socketio version

flask_socketio.emit(event, *args, **kwargs)

Emit a SocketIO event.
This function emits a SocketIO event to one or more connected clients. A JSON blob can be attached to the event as payload. This is a function that can only be called from a SocketIO event handler, as in obtains some information from the current client context. Example:


    @socketio.on('my event')
    def handle_my_custom_event(json):
        emit('my response', {'data': 42})

event – The name of the user event to emit.
args – A dictionary with the JSON data to send as payload.
namespace – The namespace under which the message is to be sent. Defaults to the namespace used by the originating event. A '/' can be used to explicitly specify the global namespace.
callback – Callback function to invoke with the client’s acknowledgement.
broadcast – True to send the message to all clients, or False to only reply to the sender of the originating event.
room – Send the message to all the users in the given room. If this argument is set, then broadcast is implied to be True. include_self – True to include the sender when broadcasting or addressing a room, or False to send to everyone but the sender. ignore_queue – Only used when a message queue is configured. If set to True, the event is emitted to the clients directly, without going through the queue. This is more efficient, but only works when a single server process is used, or when there is a single addresee. It is recommended to always leave this parameter with its default value of False.

Engine.io version

The Engine.IO server configuration supports the following settings:

Emit a server generated SocketIO event.
This function emits a SocketIO event to one or more connected clients. A JSON blob can be attached to the event as payload. This function can be used outside of a SocketIO event context, so it is appropriate to use when the server is the originator of an event, outside of any client context, such as in a regular HTTP request handler or a background task. Example:


    @app.route('/ping')
    def ping():
        socketio.emit('ping event', {'data': 42}, namespace='/chat')

event – The name of the user event to emit.
args – A dictionary with the JSON data to send as payload.
namespace – The namespace under which the message is to be sent. Defaults to the global namespace.
room – Send the message to all the users in the given room. If this parameter is not included, the event is sent to all connected users.
skip_sid – The session id of a client to ignore when broadcasting or addressing a room. This is typically set to the originator of the message, so that everyone except that client receive the message.
callback – If given, this function will be called to acknowledge that the client has received the message. The arguments that will be passed to the function are those provided by the client. Callback functions can only be used when addressing an individual client.