c-flex and bipap exhale settings

I understand, that a simple Auto Remstar C-Flex assists in exhalation. However, a bipap machine is also designed to assist exhalation by prescribing the lower exhalation pressure. I would like to know from the knowledgeable people on this site as to what is then the difference between the two.

For starters, here is a brief discussion: http://smart-sleep-apnea.blogspot.com/2 ... oices.html