Meaning of Rim specs

[Agustin de la Calle <delacall@bcmp.med.harvard.edu>]

Hi there,

Does anybody know what the letters in the rim specifications mean?
        Tire    Wheel
        155/80  4.5 J 13"
                    SD(?) or
                    SL(?)

So far I know that the 80 represents the ratio of height to width in %.
That's not much ;-)

Thanks a lot!

Agustin