The name of the file (relative to catalog root if no leading /) that maintains the order number counter. If not set, the order will be assigned a string based on the time of the order and the user's session number.
OrderCounter etc/order.number
Bear in mind that MiniVend provides the order number as a convenience for display, and that no internal functions depend on it. You may define and use your own order number routines without fear of consequences.
If you have a Route set up to supplant and the counter attribute is set there, then this is ignored.