How to modify print layout of invoices?

Hi,

I would like to modify the print layout of invoices. I use the VF03 transaction to select an invoice, then "Billing document - issue output to" to show the selection dialogue with two message types (for two output versions) and then print invoices.

Now, I suppose the layouts are generated in SMARTFORMS, but I don't know which of the forms from the list is used to generate invoice layouts?