First off, I don't think there is an industry standard, so different rod companies may use the terms slightly differently.
As an example, though, the Shimano World Shaula Technical Edition has 6'2" rods in XUL, SUL, UL and L.

The recommended lure and line ratings are as follows:
Mode Lure (g) Line (nylon lb) Line (PE line size)
XUL .4 - 3.5 1 - 3 .2 - .6
SUL .6 - 6 1.5 - 4 .2 - .6
UL .7 - 8 1.5 - 6 .2 - .6
L 1.5 - 10 2 - 7 .3 - .8