The training scale are touch stones to use in checking where errors are in training, placing value in steps. They are NOT linear, and all interact. For more information read the german FN book.
RHYTHM refers to sustaining/developing PURITY of the gaits (4 beat walk/2beat trot/3 beat canter), but it also refers to have the appropriate tempo which allows for that. And the appropriate tempo allows for a swinging back which is also known as relaxation.
SUPPLENESS refers to the use of lateral flexibility which in turn develops longitudinal balance.
CONTACT is a steady connection appropriate for the level of training. For the young horse it is acceptance of the bit, for the younger-middle level horse it is being steadily on the bit (with appropriate degrees of flexion), for the middle-upper level horse it is the development of self carriage.
IMPULSION is elastic lifting and placing of the hind legs, the development of thrust.
STRAIGHTNESS is a topic always addressed. First it is merely going straight ahead with even contact in both reins, then it becomes the horse learning to 'stand on the outside rein' and being effected by that containment (since the horse is wider behind than in front), and gradually it is the placing of the forehand in front of the quarters (i.e. In shoulder in/renvers).