Starting from the invention of the pneumatic tire, tire carcasses were reinforced with cotton cord. If there were four layers of cotton cord, it was 4-ply, etc. Forty or fifty years ago, or more, nylon cord replaced cotton cord, but the number of plies stayed the same for a while. Manufacturers started using 2 plies of nylon cord with "4-ply rating" and had to try to explain to the public that these were actually better (they were). This evolved into Load Ranges, 4-ply becoming Load Range B, also called Standard Load. The tire makers now use whatever combination of plies, belts, etc. to give the tire the characteristics and Load Rating they want to achieve.
Yes, Load Ranges, i. e., B, C, D, E, etc., are standard throughout the industry. There are also Load Index (110 = 2337 pounds, for example) and actual load capacity in pounds and kilograms molded into the side of each tire. Here's good info:
http://www.tirerack.com/tires/tirete...l/sidewall.htm.
Here's an example of Load/Inflation tables for some tires
http://www.trucktires.com/library/te...fl/table13.htm. You can get these tables for your tires at a tire dealer or from the tire maker.
Ken