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 to 
Invoice/ PaymentMeans/ InstructionNote/  Invoice/ PaymentTerms/ Note?
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.