For some of the customer, they don't want the sales order and the invoice number to be the same, so that they can keep track of the sales order and the invoice easier by differentiate the number of sales order and invoice. Here is the step to enable the config.
Step 1. Select module on setup menu
Step 2. Tab sell - Numbering - sales order enabled (Yes) then save
II. How To Use This Function
Step 1. There are two Numbers on sale menu, Invoice and Sales Order Number
If you park the sales order number will be the number 1 and if you finish the order the invoice number will be the number 2