Dear guys/gals

I am new to UBL and its implementation for invoices.

Would like to clarify if my understanding of the below elements usage is correct or not.

Invoice/ PaymentMeans / PaymentID 

I am working on an ERP system which needs to create/read UBL formatted invoices (purchase and Sales).

and i was wondering where i can put my payment reference information. From the UBL specification website (http://www.datypic.com/sc/ubl20/t-ns19_InvoiceType.html)

it seems that the only suitable place is to put it in "Invoice/ PaymentMeans / PaymentID".

Currently in my client company, they have a legacy system which also creates/read UBL formatted invoices, but they are reading and writting it to 

Invoice/ PaymentMeans/ InstructionNote/

Invoice/ PaymentTerms/ Note

Will it cause any problems, if we stored the Payments Reference there?

Or is it a hard rule , that payment reference needs to be stored in "Invoice/ PaymentMeans / PaymentID" only? 

Am i correct to think that, we can put the payment reference in any elements in the UBL, as long the receiving system can identify from the elements it is a paymente reference.

For example: If system X get Payment Reference from "Invoice/ PaymentMeans / PaymentID", when reading my client company legacy UBL, they wont get the payment reference.

This brings me to my next question, does all UBL implementor uses "Invoice/ PaymentMeans / PaymentID" to store payment reference?

Thank you in advance

Your reply is greatly appreaciated.