OnHeadersAvailable is a TIdHTTPOnHeadersAvailable property that represents the event handler signalled when processing a response received by the HTTP client. OnHeadersAvailable allows the client to decide if response processing should be continued based on headers returned in the HTTP response. 

Applications must assign a procedure to the event handler to allow responding to the event notification. Set the value in the VContinue argument to False if the response should not be accepted and processed. This also causes a persistent connection (indicated in the KeepALive property in the Response) to be closed. Use the values in the TIdHeaderList argument to the event handler to examine header values received for in the Response.