That is "Light Duty Truck" as opposed to "Medium Duty Truck" or "Heavy Duty Truck". An S10 or ranger is the very lightest of the light duty trucks but LT generally includes everything up to and including a one ton, e.g. F350 or Chevy/Dodge 3500. Officially the federal gov't ends "light truck' at 8500 lbs gvwr for fuel economy (CAFE) purposes, but that has no bearing on tire capacities.
Yes, the load range letters equate to "plies", but tires don't actually have plies (layers) anymore, so even that is a misnomer. And plies or "ply ratings" do not equate to a specific load capacity. A "G" tire is stronger than an "F" tire in the same tire series, but a "G" of one make & model" is not necessarily the same as a "g" of another make & model. It's probably close, but it is a mistake to assume all "g' have the same load capacity. they do not. Ply ratings are useful only in comparing within the same tire make & model series.