40y is a 40 grit belt with y weight backing
y refers to the stiffness/weight of the belt. X weight is the heaviest commonly available. Y is slightly lighter but it is still pretty stiff.
As Boss dog has noted it Alphabetical, Y is about as heavy/Stiff of a backing as you can get. With J about the lightest/Most flexible that is commonly available.
X and Y would be for heavy grinding and I use J weight for contouring, Convex grinding or sharpening.