The "red envelope" in GroupWise had no real function other than to give visual feedback that the item was of High Priority. You may change the message options to send it as a High Priority message which will display the "red envelope" style of vidual feedback.
Flagged items may be interpreted by the reciever similarly, though a flagged item also contains a follow up (task reminder if you will) feature along with an alarm. In this sence, it's a little more than the "red envelope" than we have had in GroupWise.