How do plated through holes/slots shows on the Gerber?
Step 2. Put milling slots in the mechanical/outline layer.
Plated holes/slots:
Circuit layer: holes have copper
Solder mask layer: holes have openings
Non-plated holes/slots:
Circuit layer: holes have no copper
Solder mask layer: openings don't matter
Circuit layer: holes have copper
Solder mask layer: holes have openings
Non-plated holes/slots:
Circuit layer: holes have no copper
Solder mask layer: openings don't matter