The more usual custom is to give the back order the same number as the original of which it is a part.
Of the two copies of the back order, one goes to the shipping clerk, while the other is kept in the office.
When a back order is filled, the shipping clerk returns his copy to the office.
an order to purchase something that is not currently available but will be sent at a later date
Back orders are held open for future delivery, usually for a designated period of time after which they are cancelled.