Load index is a code for a mass
The number in the service description - the 102 in 225/65R17 102H - maps to a maximum load for one tire in a published table. 102 is 850 kg, or 1874 lb. The mapping is not linear and cannot be worked out; it has to be looked up.
The figure is per tire, at the reference inflation pressure for its load range, on a sound tire mounted on a correctly sized rim. Four tires carry four times that, but the vehicle's own axle and gross weight ratings are usually the lower limit and are the numbers that actually govern.
Load range is a construction grade
The letter on a light truck tire - C, D, E and so on - describes how the tire is built and the maximum pressure it will hold. Load range E is a 10 ply rating equivalent with a maximum inflation of 80 psi. Higher letters carry more at the cost of a stiffer ride.
It is a different scale from the passenger tire designations. Standard load and extra load describe the reference pressure at which a passenger tire's load index is certified: 35 psi for standard load, 41 psi for extra load. An extra load passenger tire is not a load range E tire and the two are not interchangeable.
Ply rating is an equivalence, not a count
A load range E tire is often described as 10 ply. It is not built from ten plies. Modern radials use one to three plies of material far stronger than the cotton cord the scale was written for, and the number describes the strength that many old plies gave. Cutting a tire open and counting layers will not produce this number.
The two adjustments
A P-metric or Euro-metric tire fitted to a light truck, SUV, pickup or van has its rated load divided by 1.10 before it counts. That is a federal requirement and it is why a passenger-construction tire cannot simply stand in for the LT size a truck was built around.
An LT tire fitted in duals carries about nine percent less than it does singly, to account for road crown on a twinned pair. That is why dual-rated tires print two load indices: the first for single fitment, the second for dual.