By default, all printed circuit board mechanical layer and keepout layer (protel software) will be milling or cutting treatments. Do not use mechanical layer to defined any copper line or silk characters.
Defined a pcb board outline layer, You can use mechanical layer, The protel software is called keepout layer.
Defined Non-metallic annular slot, Please use the mechanical layer. Any other layer's hole will be metallized. Or may even be ignored.
If you want to add a few of non-metal hole, you can add an new mechanical layer to define. And named the new layer as "NPTH (non-plated through hole)".
Do not use silkscreen layer to defined any slot or annular sign(annular mark).
DiGPCB's manufacturer minimum annular slot distance: >= 0.8 mm, minimum drill size: >= 0.3mm (mass production)