Backing is something that is necessary to have behind the fly line on all reels. When the fish rushes and the usual fly line starts to run out on the reel, then it is good if you have backing that can handle if the fish decides to continue rushing and thus pull out more line.
For most fishing for grayling, trout, rainbow and char, a drag of 15-20lb and 50-100m backing on the reel is sufficient. On the other hand, if you are going to fish for slightly larger fish, such as salmon and sea trout, then a pulling force of 25-50lb and 200-300m of backing spooled on the reel is often required.
