Just more precise level detail for I think a sensible progression. AT might advance you from say a 5.3 to a 5.6 in the SS PL and gradually build it to say a 7.0, but it wouldn’t be sensible for AT to advance you in one go from a level 5.3 to a 7.0 without burning you out.
Everyone has covered how the workout levels work. I will add a caveat in case you start to actually compare some workouts and their levels, they’re not perfect by any means. But, TR says when WLv2 comes out this will all be fixed. So hopefully, in a few months(?) this will no longer be an issue.
Actually, the official term is “Workout Level,” which applies only to the workout. The term “progession level” applies only to the athlete, despite its widespread misuse in the forum. Minor quibble…that won’t make you faster