lights schedule and dimming interaction

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Your description leads to the documentation for Daylighting:DELight:Controls.

From the Engineering Reference, section DELight Daylighting Calculations, this description confirms that the electric + daylighting is the (total) illuminace used to meet the target:

For each point in time, DElight calculates the interior daylighting illuminance at user
specified reference points and then determines how much the electric lighting can be
reduced while still achieving a combined daylighting and electric lighting illuminance
target.

Regarding your final question, from the Input Output Reference document:

Field: Illuminance Setpoint at Reference Point

The desired lighting level (in lux) at this Reference Point. This is assumed to be the
lighting level that would be produced at this reference point at night if the overhead
electric lighting were operating at full power. Recommended values depend on the
type of activity and may be found, for example, in the Lighting Handbook of the
Illuminating Engineering Society of North America. A typical value for general office
work (excluding computer terminals) is 500 lux.

This simply means that the target could be produced ONLY by electric lighting. One might ask "how do I know if my lighting system can attain that illumination level"? That's a good question, since you could define electric lighting levels, for example, at either 3 W/sqft or 1 W/sqft. How could both of these electric lighting levels produce the same lighting level (lux)? The answer depends on the type of lighting used, which in the case of EnergyPlus, is not an input. So the end result is that the user must pay close attention to the target value to ensure daylighting is modeled appropriately (and that the reference location is representative of the lighting level target potential).