The difference between regular and optional purchase order is given below:
Regular Purchase Order: It allows you to track receipt note and/or purchase vouchers recorded against an order, therefore making it a complete cycle. Also, only regular purchase orders affect the Purchase Order Outstandings reports.
Optional Purchase Order: It will not allow you to track receipt note and/or purchase vouchers made against an order.
One similarity between both the vouchers is that they do not affect the books of accounts.