OK, I have looked around, and have missed the answer that I am sure is obvious to everyone else and staring me in the face.
The "Envelope" icon next to posts on the left. What does it indicate? Some are blue, some are red, some are read witht he envelope open and 3 pages sticking out. I thought it might be an activity indicator, but I see threads with 4 pages and infrequent posts have the "open envelope" icon, and posts with 9 pages and frequent posts have the "red envelope" icon.
Thanks for the help in advance.