The bowstring can affect the draw speed, Arrow (or Bolt) speed, and the overall durability of the bow or crossbow. There are only three possible materials for a bowstring: String (Vanilla), Enchanted Fabric (Thaumcraft), or Flamestring (Natura).
Bowstring: A plain bowstring fashioned from vanilla string will no effect whatsoever on the resulting ranged weapon.
Enchanted Bowstring: penalizes arrow speed by 10%, resulting in arrows or bolts with lower damage that move slower and drop further before reaching their target. However, it grants Stonebound (increasing draw speed as durability decreases), a 20% decrease in draw time (another boost to draw speed), and an extra modifier. The resulting ranged weapon does not report this additional modifier with a name (like Thaumic or Writable), and this special additional modifier stacks with any other material that grants additional modifiers without conflict.
Fiery Bowstring: penalizes draw speed by 10%, but offers a 20% boost to arrow speed (increasing impact damage), as well as a 20% boost to the ranged weapon's overall durability. This is the best choice of bowstring if the bow or crossbow easily makes it to the 0.25 draw speed cap through its other materials, as the boost to arrow speed is invaluable and difficult to find elsewhere. This bowstring does not add a flame effect in any way, shape, or form.