We already discussed that in a previous post.
For auto invoicing the number can be used as reference
I do not think they should be booked using invoices but with Expense reimbursement
I see there are at least three implementors (virtual-things, datalife, nantic) which already have some custom code to make it required. Won’t be better to just make the field required on base (only for supplier invoices) and let who does not want such behaviour customize?
For the normal usage, the supplier invoice reference should be always filled with the supplier reference.
At least for Spain as we use such field for reporting the invoice to tax authorities