Hello there, svbapatandco.
I'd like to help you with your concern. When you say post a unique document no. are you referring to the custom transaction numbers? If so, you can enable the option by following these steps:
Here's a sample screenshot for a visual guide:
You can also read this article for reference: How to show transaction numbers on sales forms.
If you're referring to something else, please feel free to add more details in your reply. I'll be right here to help you out again. Thanks.