The Circuit Scribe pen has a ball-point tip, so it writes best on surfaces with paper-like or cardboard-like texture, and conversely does not easily write on glass, plastic, or FR4.
Regarding writing directly onto a printed circuit board (PCB): If you want to attempt basic rework, like connecting a component to an exposed pad or exposed trace on the PCB the CS pen can be a temporary fix for a bad solder joint. Drawing a completely new trace across the top (smooth) surface of a PCB is not a recommended use for the CS pen.
You can use it to connect passives to their pads, but while the ink has very low resistivity it also has very little mechanical strength. Therefore the CS ink is not a 1:1 replacement for solder. If your goal is to electrically conduct and bond components down we recommend using an adhesive to hold the component down.
An example of where the CS pen would be handy when doing PCB rework: if you have cut a trace on a PCB you can fill in the gap with a dab of the CS pen. It is not recommended to use the CS pen for high current circuits.
Automotive or Milspec?
The CS pen is not intended to make PCB repairs that meet Automotive or Milspec.