Edit AR invoice Special Instructions

Summary

"Special Instructions" box at the bottom of invoice - how is it edited?

Content

Our AR Invoices show a box at the bottom, below the "Send Payment To" remittance instructions, called "Special Instructions."

How is this edited?
Can it be edited specifically for one customer?  If so, can it be added more or less permanently, and not have to be re-added with each invoice?

Thanks,

Beth