I think it might be:
http://www.teamviewer.com/en/help/216-What-is-the-difference-between-AddOn-workstations-and-AddOn-channels.aspx
"The number of channels determines from how many workstations sessions can be started concurrently. E.g., the Premium license has one session channel and an unlimited number of installations. That means that if one Meeting or multiple Remote Control sessions are started from one computer, no second computer would be able to use TeamViewer at the same time."