The sales order line number is used if there are multiple sales order items on one sales order

It is easier to keep track of the multiple items on an order if line item numbers are used.  When a Sales Order Item is entered, the next sequential line number is assigned automatically but can be overwritten if required

For example, if the 4th line item of an order was entered first, the system would allocate line number 1 but this could be overwritten to line 4 by the user entering the order item