(last updated: 12/28/17)
In any fight, flexibility is key. Largely representing the physical acumen and/or technical competency of a fighter to deal with manipulation attacks such as joint locks and takedowns, Roll is a fighter's resistance and ability to avoid injury when fighting on the ground or in tight quarters.
Throw covers a lot of things, including abilities such as worming your way out of a grip, and breakfalls. However, while Throw competency itself largely deals more with the technical aspect of fighting up close and personal--that is, a fighter typically has to train to be good at throwing--Roll can be just as easily a measure of a character's base flexibility as it can be a measure of their technical ability. Some people, whether through directed training, nature, or just plain diet, are naturally hardier to being tossed and bent up than others!
At extremely high masteries of Roll competency, a fighter may in fact have some of the most bizarre physiology seen in the world, and the feats of Roll masters frequently feature on some of the most impressive and often-shared fight videos. Your body is oftentimes controlled entirely at your will, and it's not uncommon to see such a master bend, stretch and contort their limbs into any number of anatomically improbable shapes. Some may even have partially liquid anatomies entirely, making their joints and weak points extremely difficult to target.