Thank you, this does not answer my question. I'll try to be more clear. Each invoice has a Purchase order number. When I send statements, instead of including only invoice number, I need to include purchase order number, as this is what most customers use. Currently, I am running a report with open balance by customer and including PO as a column, then individually sending each report to each customer. You can imagine how time consuming this is. 

I have investigated "manage templates" and customizing the statement template. I do not see any option to include PO. Seems like a very basic requirement which should be standard in any accounting system.