We use the Reference field on the Sale Order to record the customer purchase order number. As such, we are adding this field on the invoice.
I imagine we are not the only ones?
If so, would it make sense for this to be in the Tryton sale module?
I imagine this might be one of those things where the Reference may not be used for this purpose in some implementations and thus it might not make sense for this field to be surfaced on the invoice.