What settings should I use to produce power plane Gerber plots

DesignSpark PCB provides several options for the generation of Gerber plots for power planes and you must select and configure the options to meet your manufacturing requirements.

An overview is presented here:
DesignSpark Article link

From the above link you will observe that the following is the modern recommended setting:

Positive (plane and items): Produces a full plot of everything that is required as copper on the finished board. This is the normal recommendation for most manufacturers.

History of power planes.
Power planes for a PCB are another area which has features from many years of different techniques.
Historically the outer layer copper track layouts were produced using taped tracks at 2x or 4x scale and then photographically reduced.
The power plane layer images were derived from the pads on the outer layers and produced photographically and due to the process they were produced as negative images. This method also restricted copper tracks from being placed on the power planes.
With PCB CAD power planes initially followed the 'artwork' methods as the file size was important and a negative image power plane has a much smaller file size.
Tracks on power planes were also then possible to produce but it is not recommended as it will reduce the efficiency of the power plane.
If you wish to have a power plane layer with tracks or a split power plane the recommended method is to use inner signal layers with copper pours for maximum flexibility.