It was my understanding that when a direct drive hubmotor is shown on its side, the "H" is for the "Height" of the lamination stack. If you visualize the hubmotor in its vertical normal orientation when it is mounted on the bicycle, we might call it the "width" of the lamination stack. It designates 35mm wide. When the copper phase wire is wrapped around the stator tooth, it becomes an electromagnet that can be turned on and off. The width of that electromagnet determines how much power it has.
The permanent magnets in the rotor (motor rim) are typically exactly the same width as the stator. However there have been several times when a motor was found to be a millimeter or two wider than the magnets.
The 35H hubmotors are the most popular direct drive style, because they will fit on a normal bicycle frame with the common drop-out width of 135mm, and they will also still easily fit a 6-speed freewheel / derailleur system (and sometimes a 7-speed).
The narrower 28H direct drive hubmotors are usually only found in the cheapest ebay kits. They have thick laminations and thin phase wires, and they can only only survive modest power levels, unless you do something to release the excess heat that builds up. The quality is typically average at best. If you are limited to 2,000W or less, then you might be better served by any one of the several large geared hubmotors. They provide good torque from a smaller and lighter motor, as long as you don't overheat them.
The 35H motors are appropriate from around 48V X 50A = 2500W, up to around 5,000W peaks (as long as you keep an eye on the temps). If you add ferro-fluid, that helps it shed heat, and you can apply even more power.
If the 35H motors are overheating in your particular application, then you might need a bigger motor. The most popular next size up is the 45H MXUS 5K turbo. As the name implies, it is rated for 5,000W
The QS and Cromotor are both 50H (if I recall correctly), but that also means they are heavier and more expensive. If you are looking at 35H hubmotors, I highly recommend getting two well-made torque arms. If you do not, you will likely regret it it later.