Reply is a TIdRFCReply property that contains the NumericCode and Text that makes up an RFC response message. Reply is given the value in the ReplyNormal property of the command handler, when it is created by the command handler. 

Reply can be altered if an exception is raised during execution of the command. After an exception is raised, Reply will contain the ReplyExceptionCode, from either the server or the command handler that raised the exception, and the exception text. 

Reply is sent to the peer connection prior to the Response for the command handler when PerformReply is True.

TIdRFCReply 

TIdCommandHandler