Bruno, it works like this.
Lets says you attach a Z output to a sales order which sends the details of the SO to the customer in a PDF format. For this purpose you have developer the o/p using smartforms or scripts. Now there are few configs that you will perform in order to send the PDF to the customer AUTOMATICALLY once the SO is saved. This is done thru the transaction NACE where you will attach the smartform, the o/p, partner function and driver program.
This driver program will get kicked in when the order is saved and will perform the logic and then call the smartform.
Hope this give a little bit better picture.
Vikram.M