There's also a lot of good info right here in FGRV's document center (link at upper left of the home page under "resources")
I found these docs real helpful in trying to understand the difference between the tire types.
Here's an explanatory paragraph from one of the docs listed there under "tires":
(from Trailer Tire Facts - Discount Tire
in docs center)
Why Use An "ST" Tire
"ST" tires feature materials and construction to meet the higher load requirements and demands of trailering.
The polyester cords are bigger than they would be for a comparable "P" or "LT" tire.
The steel cords have a larger diameter and greater tensile strength to meet the additional load requirements.
"ST" tire rubber compounds contain more chemicals to resist weather and ozone cracking."