Sorry Paulie, the PM was to Rajangan.
Air curing is done slowly in the shade, but no added heat. Sun-curing is obviously done more quickly using the sun to dry it out. Flue curing is a controlled cook of the leaf.
Air curing will give burleys a deeper robust flavor, while sun-curing can be harsher. Bright leafs can be cured by these methods, but never reach a truly sweetened state, and are more earthy in flavor, but no hay-like or citrus notes.
Bright leafs have a lot of carbohydrates in the leaf, think "tomato". Tomatoes are tasy off the vine, but if you stew them, they turn the carbs to sugars and get sweeter, and as you keep cooking them, they get even sweeter.
Anyone with more knowledge, feel free to correct me...
The leaf is still alive when it is picked. And, even dried it is still a living thing full of ammonia that makes it harsh to smoke. Any tobacco is going to need to be sweated or socialized, curing it to gas off the ammonias and other living chemicals that cause harshness. Aging in a barn was how my family used to do it. Sometimes, even burying the crop to keep it warm during the cold North carolina winters. Some people will cavendish the leaf. Stacking and allowing the natural heat of the leaf to cure it is typical also. But bright leafs were developed to give a sweeter smoke using controlled heating, and color curing for a quicker turn around. Mark Ryan told me that typically three years was the amount of time farmers would wait for a natural sweating of the leaf. Pressing into plugs, twists, toasting, Blackeny's processes, cavendish, or just casing them, blending them, or even fire curing are commercial ways to speed up the process of making a smokable product.
All in all, there are many variations that can make a leaf more palatable. And, apparently, none are really wrong, unless a barn load gaums up or molds. But, this is also how a seed stock called Virginia Gold can end up tasting like 5100, FVF, Brown Flake, Tilbury, LTF, Virginia #1, Kendal, or Hombergers. Such a huge variation in flavors from one plant seed. All top grade leaf, just processed differently.
Rustica... I grew rustica my first year. It is a neat prehistoric plant, but I ended up using just a smidgen of it in blends, because it is SOOOOO powerful. As an air-cure without aging or color curing, it made me sick, and I don't get sick easily from smoking. I suspect Cotton Boll Twist is aged very well before selling and it is still the strongest thing I can smoke. So, if you just want to "ride the dragon" that one would do, ha ha.