Proforma invoice generating

I want to understand something in proforma invoice…
According to the documentation and with my test proforma invoice can be generated when a lead is converting to a customers and in this case the deal begin a proforma invoice or invoice depending on settings.But when i choose a client and i want to creat a proforma invoice in the list of service i can only choose one time plan and not internet or voice plan.
My question is in when the proforma invoice can be generated for a customer if i want to send him now?