KiCad how to export oval PCB hole as a drill file?
Round holes in the board are made using drill bits in an automatic machine that pokes at the board with a spinny thing,
The DRL file gives the locations and sizes for theses holes. if you make two holes overlap there's a large risk that the drill bit will break so that is against the rules.
slots are made using milling bits in an automatic machine that pokes at the board with a spinny thing and then drags the spinning cutter along a defined path.
2mm is the diameter of the cutter usually used so slots narrower than 2mm, or with corner radiuses less than 1mm may cost extra. Smaller cutterrs are available but because they are more easily broken the cuiting speed is reduced, and because time is money you will pay for that.
PCBWAY asks for the slots to be included on the &quot;Board Outline&quot; Gerber layer,
check with your chosen manufacturer on how they want slots presented.
Sometimes you don't need slots, eg CUI makes a 5.5x2.5mm jack with &quot;round&quot; pins, intead of flat tabs.