E-rated is referring to being a tire constructed for 80 PSI maximum pressure; which is considered 10-ply
rated. Being that we're talking radials and not bias ply tires, the archaic ply lingo is a bit strange, but that's what the tire industry has held on to.
General tire info: If you take a given tire size (lets say, LT265/75R16 and compare a C rated (max press. 50 psi, 2470 lb max) to a D rated (max press. 65 psi, 3000 lb max) to an E rated version of that tire, the E rated tire will be rated for more weight (3415 lb max)
at its maximum pressure. Keep in mind, at a given pressure (e.g., 50 psi), they all are rated for the same weight. In this case that's 2470 pounds. The higher load capacity of the E-rated tire is because an E-rated tire is capable of handling the higher inflation pressure required to bare that higher weight. In order to be able to handle those higher pressures, E-rated tires tend to have more rugged construction, which incidentally enhances off road durability at some sacrifice to tire flexibility and total weight. How much more durable, I don't know? I find the load inflation tables useful, here is Toyo's:
https://www.toyotires.com/media/2125/application_of_load_inflation_tables_20170203.pdf
They're great tires, but not perfect. They sucked in mud and they're not so cheap these days. I got the new Toyo's for less, as there was a sale. Falken's were a bargain when I bought them, but with a Toyo being cheaper, looking to be better in mud (greater void space), and having a 3 ply sidewall (Falken was only 2 ply sidewall) it was an easy choice. Plus, I'm always looking for the next best tire.
From what I can tell, the AT3 is an improvement on the AT2. May be one to watch going ahead.