Can anyone explain the wheel dimensions as far as the end letter code goes?

The sticker inside my door jam has info on the tire size, psi, and rim size.
what I don't know is what the letter at the end stands for.
For example...

My tire size is 285/75/16 C
My rim size is 7.50 X 16 J

I have a trailer that has rim size 5.50 X 16 F

What are these letters on the ends of the sizes? C, J, F ???