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.
Background of other options:
Power planes in PCB CAD is another area which has features covering many years from the original taped layouts at 2x or 4x scale which were then photographically reduced. The power plane images were derived produced photographically from the other layers which is why they were produced as negative images and also why no copper tracks were allowed.
As tracks on powerplanes evolved as a requirement, the other plot options were added, but as stated in the article link above will rarely if ever be required for modern production methods.