Control transformer 1 (CT1) is what the % rated current meter is measuring. CT1 measures all windings, depending on position of S8.
NEVER run any load over 500 Watts, even though the circuit breaker is 15 amperes which will support a 1500 Watt load. Why? Because the outlets are driven by only one of the six windings. So even though the generator can put out 52 amperes at 120 volts, the outlet can only deliver 1/6th of that or 8-2/3 Amperes (8.66 amps).
When one winding is loaded, the control voltage transformer along with the voltage regulator boost field voltage to compensate. This causes the other unloaded windings to generate higher voltage which can break down insulation. Running 15 A off the outlets also heats the one loaded coil and thermal expansion conspires with the voltage stress to break down insulation.
When the windings are uniformly loaded, the heating is uniform and very little thermal stress happens.