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.
Bear in mind that Interchange provides the order number as a convenience for display, and that no internal functions depend on it. Custom order number routines may be defined and used without fear of consequences.
If a Route is set up to supplant and the counter attribute is set there, this is ignored.