On the Pirelli Diablo Stradas they used to call it an "E" code.
Some may ask, what is different in these alphabet soup tires as compared to the "regulars?" In most cases (all that I know of) they only add an extra ply in the tire sidewalls.
What does that do for you? Well, a stiffer sidewall will reduce sidewall flex with a given load. That means the tire will not heat up as much with all other things remaining equal (load, speed, inflation pressure and ambient temp). Excess heat is the biggest enemy of any tire. You need to have some heat to get the rubber flexible enough to conform to the road surface and provide good traction. But it is a trade off as heat causes accelerated tread wear and can (and will) eventually result in structural damage and a blow out.
You can improve the tire's ability to perform with a heavy load simply by increasing inflation pressure, but only up to the maximum pressure rating for the given tire. Higher pressure has the same overall effect as the extra sidewall belt in reducing sidewall flexing and thereby reducing heat, but it also makes the tire tread area harder and you lose some contact patch and traction by going too high.
So, to summarize, the added sidewall gives you stiffer sidewalls, for improved heat management with heavy loads, but at a lower (more normal) inflation pressure than the normal tires. If you do not ride with a heavy load very often they will not do much of anything for you except lighten your wallet.
edit - On reading what I wrote, I realized that a person could order the alphabet spec tire and then run that tire at a reduced air pressure (with a non-heavy load), which would allow the tire tread to squish flatter on the road, increase the contact patch and they might get better traction as a result. I dunno...
