Fly line backing serves two basic purposes. The first is to fill up your reel so that your fly line completes the filling of your reel to within, say, 1/8th inch of the spool capacity. The second is to still stay connected when a big fish strips of all your fly line!
For most purposes, our Airflo No Frills backing supplied in 100 meter spools is perfectly adequate as a trout backing line. If you require a stronger line, say for salmon fishing,then you should consider another backing line on this counter.