Workout Designations

[Rudeboyelvis]

Noticed in the 8th race at Saratoga the latest work for the 3 horse, Original Six,
was classified as "D". According to the closer look, this means "driving". I can honestly say I've never seen this before. I'm assuming that this translates into a much stiffer ride than "handily". So the question begs, exactly how many workout classifications are there? and does Driving mean what I'm assuming?