usually the order reference ID is the session ID.
this gets set on the pp_confirm.php page in the store order summary behavior.
if you wanted to use something else you would need to come up with a way to generate a unique number for the order reference ID.
you could use the Security Assist Random Number binding to set a session variable with a random number, but there is no guarantee the random number would be unique.
the shorter the string, the more chance there is of duplicating the random number.
