This is about the red charge light on the headlight shell just behind the speedometer.
I like to look at it this way: the red indicator is a 6V incandescent light that is connected between the battery and the generator. This light bulb burns at its rated value when there are 6 V across its terminals. That is why it lights up when the generator is not generating. It then, effectively, is connected to +6V from the battery and 0 V from the generator, so it lights up.
When the engine is running fast enough, and the generator puts out +6 V, the indicator has +6 V on one end and +6 V on the other: no voltage difference, so it doesn’t light. However, if the generator output does *some* power, but the voltage isn’t adequate, the voltage difference across the indicator will be smaller than 6 V. It might burn softly, but it will be very hard to see. This is typical for light bulbs: if connected to half their rated voltage, they burn at 1/4 power, and thus approximately 1/4 light output. Do the math, and if connected to 1/3 the voltage, it will burn at 1/9th power. It will be very hard to see, especially when there is ambient light, as on a sunny day. From this one learns: the red light will indicate that you are using battery power, for example, when you are about to start the engine and have the key pressed in. It will indicate a major failure in the charging circuit of your bike.
It will NOT indicate discharge during riding the bike when the generator/regulator is “just not up to it.” You might be slowly depleting the battery, and the indicator will not light.
Updated 1 Nov 2019