A ``Critical Message'' is a message that will appear immediately after the
recipient user logs in. The user must acknowlege receipt to continue.

To send a Critical Message, push the \textbf{COM} button on the remote
to get to the Communication screen. On each of the message sending
screens (\textbf{Send message to user(s)}, \textbf{Broadcase message
to course}, and \textbf{Upload message to course}), there are two
checkboxes, \textbf{Send as critical message} and \textbf{Send as
  critical message and return receipt}.

\textbf{Send as critical message} will send this message as a critical
message. \textbf{Send as critical message and return receipt} will
send the message as a critical message, but additionally return a
message to you each time a user acknowleges receipt of the message.