An error code.
You should always accept a connection request. Once the connection is made you can check the client and close the connection if you do not wish to maintain it. Since the connection requests are queued, if you do not accept a connection, it remains in the queue. When the next request is received, it is placed in the queue behind the first request. Calling Accept at this point will connect the first request, not the second, since the first request is still in the queue.
For an example, refer to SendAssociateRequest.