I'm sure someone else can explain it better, but a long time ago, WDW sold ticket books instead of MYW tickets. The books had tickets that you tear off for each ride, and the rides were rated A through E. E-ticket rides were the major headliners, like Space Mountain, and the same term is used now to describe the most popular rides (like Soarin', TSM and Expedition Everest), even though those rides were built long after the ticket system changed.