For boards that do not require solder mask at all (exposed traces and bare FR-4 substrate material), then the solder mask layer should be completely filled like this:
If your boards do not require solder mask, please do not exclude the solder mask layers from your upload. The solder mask layers are mandatory for all PCB orders. We will assume that you have forgotten to export them and ask you to re-upload your files.
Here is a PCB.
If you want no solder mask in your PCB, you can draw the solder mask layer like this.
If you want your PCB all covered by solder mask, you can draw the solder mask layer like this.
Tip: Please apply the outline to all layers when generating the gerber, if the solder mask layer has no pattern, it may cause error information in CAM software.