It consists of 6 numeric characters and must be unique for each transaction for a given shop on the same day.
Note: the uniqueness of the transaction identifier is based on the universal time (UTC).
The merchant website must guarantee this uniqueness during same the day. It must be between 000000 and 899999.
The numbers between 900000 and 999999 are reserved for refunds and operations made via the Merchant Back Office.
Input and output field, returned in the response (IPN and Return URL).
The form is rejected: