Generating label for pickup

Is there a way to generate label (QR code or barcode) in PDF which can be printed and attached on a package using Stuart API?

I found a post mention the similar thing but still don’t know how it actually works.
https://community.stuart.engineering/t/scanning-feature/607

What is the different between client_reference and access_code? And what is the role of access_code in pickup and drop off?