Board Thread:Dark Souls/@comment-216.185.249.240-20141213014004/@comment-5264386-20141213015653

The letter of the parameter bonus is simply a representation of the scaling value the weapon has. For instance, if a weapon has an E rating in Strength, then it scales poorly with Strength, no matter what your Strength stat is. In other words, the scaling rank of a weapon does not change with your stats, but rather how much the weapon has been upgraded. You can check out the mathematics behind it on the Scaling page.

Hope this helps! :)