You can read this topic for detailed definitions of the required parameter group.
RFC 2877 and the more general Telnet negotiation RFCs also allow for control characters to appear within the VAR/USERVAR variable names or their associated values. This is allowed through the use of the ESC character X'02' and rules that apply when the ESC character itself or Telnet IAC control characters must appear in the negotiation sequence. Refer to RFC 1572 for a more complete description of control character escaping rules.
While the environment options buffer will show negotiations by the client, including passwords, Telnet will always overlay any clear-text or encrypted password values in the buffer to avoid security exposures.