To be plottable (capable of rendering on a photoplotter), a font must support mirroring, rotation, and scaling. There are two categories of layers as they relate to the ability to plot a font.
System Layers
System Layers text does not have to be capable of being plotted. The stackup, service, and user layers are system layers.
Plotter Layers
Plotter Layers text must be capable of being plotted. All layers that are not system layers are plotter layers.