How to access [Reference] and [Related] documents of any DocType?

I am trying to create a custom print format for Packing Slips. For this, I need to access delivery note and sales invoice details. While accessing the Delivery Note was straightforward, I could not find a way to fetch documents referenced in the [Dashboard] section of the Delivery Note. The Dashboard area has Related and Reference listings pointing to Sales Invoice, Packing Slip, Sales Order, etc. How can I access these documents in the Custom Print Format using the Delivery Note as a reference document?

May this Unable to use frappe.get_list in custom print format would help.

Thanks. This helped