Warehouses have the ability to request a unique PO/BOL/Reference Numbers for carriers. This is done through the Org Settings tab. 

For PO validation, please check this article. 

For a unique PO/BOL/Reference Number, you must use the Reference Number on the Org Settings instead of the custom appointment fields. 

The custom appointment fields tab does not support uniqueness. 

To set this up:

1. Go to My Organization
2. Access the Org Settings
3. Select the Reference Number drop-down 
4. Select YES on Include Reference Number field on all appointment forms?
5. Add a Display Name
6. You may add a Helper Text (not mandatory)
7. Select YES on Require Unique Reference Numbers
8. Click Save Org Settings