Nylon braided fly backing line, vibrant color for high visibility.
Thin material for more reel capacity.
Extremely abrasion-resistant, virtually no stretch.
Very low coefficient of friction won't damage your fly rod guides.
A good compromise between thinness and the ability to make blind splice loops.
Backing line has a fairly simple task to do; sit there on the reel so that when you catch a fish or start casting long distance you don't need to worry about the short fly line running out.