@mauroskov, you are resurrecting a topic already discussed at length many months ago. Sales order references used to be free-text fields, unlinked to anything else in the program. So you could enter your own sales order number or a customer’s purchase order number, or your daughter’s birthday.
When status tracking was added, more than a year ago, that free-text field was eliminated, replaced by the current linked field. The need to track status through invoicing was the motivating factor, as @eko has explained. A side effect of that is that the sales order reference number is now only for internal numbers. If you want a field for customer’s numbers, you must create a custom field.
Some of the relevant threads are: