The slots and holes is an important part of the circuit board design, To allow PCB manufacturing factory understand your PCB slot for intent of design is very important.
In a few cases, as the slots hole size is very small, or the slots placed in between the SMT Pads position. Please make a sign and clarify in the remarks column of order form.
By default, If any of the drill holes, slots located on the PCB copper foil, Will be plated (metallized).
If you want to add a few of non-metal drill hole to copper foil place, you can add an new mechanical layer to define. And named the new layer name as "NPTH".
If you want to add a non-metallic slot in circuit board, Please use the mechanical layer or keepout layer.
You can draw a line width of 0.8mm to defined as "slot" in the mechanical layer. Or draw a "box" of any shape in the keepout layer.
Do not use the silkscreen layer to design a slot. If have several slot and complex designs. You can attach a picture and mark the appropriate location.
Note: For non-metal drill hole located on the PCB copper foil, this very easy to misunderstood. The best way is to make a clear marker.
Usually for printed circuit board manufacturing, any annular slot of pcb board is using "milling process" rather than drill. The "milling tools" in diameter larger than "drilling tools". Use the "milling tools" can quickly deal with any long annular slot or big drilling holes.
NOT use the "drilling tools" for any pcb annular slot is prevent drills tools damage.
DiGPCB's manufacturer minimum annular slot distance: >= 0.8 mm, minimum drill size: >= 0.3mm (mass production)