Does response from OpenApi3.0 callback return to the endpoint configured with callback or client

If I am having a client hitting a request to an endpoint E1 which is configured to the callback endpoint CE1, then the response of the callback endpoint should be returned to the client or the E1 endpoint.

I am a bit confused.
Is it that the callback endpoint CE1 is invoked asynchronously by the service provider for E1 and the response from the callback endpoint CE1 is recorded by themselves and the client is nowhere in the picture as of now. 
Is it so that the invocation of CE1 be part of serving the request for E1(CE1 chained after E1) ?