OnTelnetCommand is the TIdTelnetCommandEvent property representing the event handler used to respond to TELNET commands and status changes received by the TIdTelnet client. 

Applications should assign a procedure to the event handler in order to respond to specific TELNET commands, or to perform TELNET options; i.e. tncNoLocalEcho, tncLocalEcho, or tncEcho. 

OnTelnetCommand is used when data is read using the TelnetThread, and the TELNET state and protocol negotiations indicate the command in the TELNET buffer should be executed.

TIdTelnetState 

TIdTelnetCommand