That's correct, R = resistor. The original design would run just fine on non resistor plugs, and mow all day long so long as you never tried to mow in reverse. I don't know if they modified the shielding on the module for later models, or stuck with advising owners only use resistor plugs.
Dose not the resistor plug just effect the length of the time the spark shoots? A non resistor plug will blow all the spark all at once but the resistor plug will hold it back until it is all released at once in a dense shot. As it heats up the resistance gos away do to the heat in the plug. Resistor plugs help with cold starts on choke with a dense air fuel mix.
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