The order/change of the PO number must be made at least one week before the desired date.
The PO number can be valid indefinitely or for a fixed time period, in which cases the validity of the PO number can be a full, half or a quarter of one year.
The PO number is printed on the front page of the bill under "Purchase order". The cost pool number can be added to the connection of the PO number. It will be printed on the front page of the invoice, section "Cost pool".