The Terminal-Type option MUST use the terminal type names officially defined in the Assigned Numbers RFC [INTRO:5], when they are available for the particular terminal. However, the receiver of a Terminal-Type option MUST accept any name.
RFC-1091 [TELNET:10] updates an earlier version of the Terminal-Type option defined in RFC-930. The earlier version allowed a server host capable of supporting multiple terminal types to learn the type of a particular client's terminal, assuming that each physical terminal had an intrinsic type. However, today a "terminal" is often really a terminal emulator program running in a PC, perhaps capable of emulating a range of terminal types. Therefore, RFC-1091 extends the specification to allow a more general terminal-type negotiation between User and Server Telnets.