I can say out of former knowledge and expirience that the German Training Scale is at least for dressage or driving CDE's is very helpful. It explains when and why some exercises should be choosedtoimprove the horse and why other stuff won't work. If you think it over you will have expirienced it all as well, but you couldn't put a name tag on it.
For example if the horse is still struggeling with the weight of the carriage and put his whole weight in the harness to pull you, you wouldn't expect him to search for the reins and to bend well. This is why his rythm and suppleness aren't there. A horse struggeling with weight for example will allways trot long - short and will not have an even rhythm. Because rythm is before suppleness and searching for the bit, without rhythm he can not bend well or he can't tug his nose in.