I want to know the difference between RAS Sessions and RDP Session.
RAS Session starts when domain user connects to RAS Gateway using Parallels Client, RAS HTML5gateway or RAS WebPortal. Then RAS Gateway connects to PA. Publishing Agent(PA) verifies user's credentials in AD and lists allowed publishing applications and remote desktops on the Client side.
RDP Session starts when a user launch a Published Application or Remote Desktop. All user's activity is performed on a Terminal Server and mirrored to the Client side using Parallels Client, RAS HTML5gateway or RAS WebPortal. More about RDP Session you can find in the following TechNet article: https://msdn.microsoft.com/en-us/library/aa383496(v=vs.85).aspx
Review diagram below for better understanding.