Load Index is a Euro rating.
Load Range is a US rating
Going deeper, 6 ply or 8 ply or 12 ply doesn't mean that's how many layers there are any longer. There may be only one or two layers but it is as thick as the old school measurement of x number of plies used in bias ply tires in the good old days.
Michelin LTX A/T2 Tire LT245/75R16/E Walmart $179 Load Index 120 (what does the E at the end mean?)
E means Load Range. Yes they use both Index and Range in that description.
LT-Metric, LT-Flotation and LT-Numeric Light Truck Tires
Load Range/Ply Rating/Abbreviated/Maximum Load Pressure
B 4 B 35 psi
C 6 C 50 psi
D 8 D 65 psi
E 10 E 80 psi
F 12 F 95 psi
So load range E has a ply rating of 10, is abbreviated with the letter E and has a maximum air pressure of 80psi.
The first letters of a tire indicate it's use. P for passenger, LT for Light Truck, ST for Special Trailer.
For comparison to the above chart
Euro-Metric Commercial Vehicle/Van Tires
Load Index Maximum Load Pounds Maximum Load Pressure
116/114 2,705/2,600 69 psi (475 kPa)
118/116 2,910/2,755 75 psi (525 kPa)
121/120 3,195/3,085 83 psi (575 kPa)