TIdCmdTCPClientBeforeCommandHandlerEvent = procedure (ASender: TIdCmdTCPClient; var AData: string; AContext: TIdContext) of object;

TIdCmdTCPClientBeforeCommandHandlerEvent is the type for event handlers triggered prior to passing data to command handlers for the TIdCmdTCPClient. 

ASender is the TIdCmdTCPClient for the event notification. 

AData represents the data read from the peer connection by the listener thread in AContext. AData is a variable argument, and can be updated in the event handler prior to executing an associated command handler. 

AContext is the TIdContext representing the listener thread for the event notification. 

TIdCmdTCPClientBeforeCommandHandlerEvent provides access to properties and methods in both the client and the listener thread, as well the data to be sent, prior to calling a command handler in the TIdCmdTCCPClient. 

TIdCmdTCPClientBeforeCommandHandlerEvent is the type used for the TIdCmdTCPClient.OnBeforeCommandHandler property.