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When I say throttle percentage, I refer (in brief) to how much you have the gas pedal pressed. Throttle works differently in gas and diesel engines:
In a gas engine, when you press the pedal down, a butterfly valve opens wider to let in more air. The computer then adds the proper amount of fuel to burn with the air you're letting in, keeping the air:fuel ratio the same.
Most diesel engines don't have a butterfly valve for throttle, because they don't need a constant air:fuel ratio. Instead, they simply take as much air in as they can, and when you press the pedal down, it adds more or less fuel.
This is how you control how much power your engine makes - by controlling the amount of air and fuel that get mixed and combusted. More air and fuel = a more powerful combustion event. As RPM goes up, you have these combustion events more often. You can increase the total power produced (and thus fuel burned) by either making each individual combustion event bigger, or raising the RPM.
Higher RPM causes more wear on the engine than adding more fuel to the cylinder.
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