Limited by constraint, restraint or sustained muscular contraction. A horse that is being held together, held back, forceable pulling back on the reins, or a rider using hard or physically tight hands. This is when the reins are stretched so that they form a straight line from the riders hand to the horse's mouth. This term is seen more often in Breeding Classes than in regular dressage competitions. In dressage if is addressed only when it affects the purity or the quality of the horses gaits.
Correctness refers to the straightness of the horses legs.
Horses that paddle, ring thier hocks, or are winging would not be demonstrating proper correctness. Lack of proper bend or hauches left or right, while performing lateral movements. Lack of straightness or directness on the line of travel. Coming down the center line weaving would be demonstrating crookedness. This can also be called cross-firing.