The AFTM (Association of Fishing Tackle Manufacturers) is a system of a assessing the correct weight of line to be used on a given rod. The first ten yards of fly line, excluding the first two feet of level tip, are weighed and given a number according to the following table.
|AFTM No.||Weight (in grains)|
The true relevance of this table is that it is possible to load the rod by the same amount, with differing AFTM weights of line by aerialising different lengths. Ten yards of AFTM #6 equals the same weight of (and therefore the same loading as) seven and a half yards of AFTM #9, which equal the sixteen yards of AFTM #3. All having the same effect on the rod.
If you aerialise much more line than ten yards of the weight your rod recommends then you may be overloading the rod. There are two effects to this; one is that the springiness in the rod becomes soggy and the line doesn't cast efficiently, the other is that you make break the rod. Both are undesirable.