Currently the order number is always an Integer number.
You can specify where the order number can start from (for example if you are moving old site to sitefinity ecommerce you can specify that your orders will start from 1000 instead of 0. But that's all you can do.
Also keep in mind the maximum for OrderNumbers is 2,147,483,647 currently.
In order to specify where the next order number should start from you need to update your SQL database there is a table sf_ec_next_order_number which is used like a SEQUENCE to return the next order number. So you have to execute an SQL query like :
UPDATE sf_ec_next_order_number SET next=1000;
It is not possible to use a custom pattern for order numbers.
If you need future assistance let me know.
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