Click System Configuration
Click on the small arrow beside General Settings to expand the dropdown
Click Manage Document Prefix
Click Generate Prefixes
You will see a list of Document Prefixes:
A: Delivery Order
Double click on Delivery Order
Pre-fix Status: Select Manual Key In under the drop down
Manual means you will be allowed to enter your own DO / Invoice / Credit Note Number.
Default means system will auto-generate the Invoice Number, Delivery Order Number and Credit Note Number.
Once done, click Save
Once you have configured the Document Prefix for Credit Note, Invoice and Delivery Order Type is changed to Manual - meaning you will be able to enter your own
number for the stated function (Credit Note, Invoice, Delivery Order)