RAS Session starts when a domain user is connecting to RAS Gateway using Parallels Client, RAS HTML5 Gateway. 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 launches a Published Application or Desktop. All user's activity is performed on a Terminal Server and mirrored to the Client side using Parallels Client, RAS HTML5 Gateway. 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 the diagram below: