I tried: I think I understand that it is also imposed on professionals.
In summary: each invoice exceeds 3000€ taxable (issued in 2011) must be declared to the tax by providing the customer/supplier data and the amount. I think it's a good thing to allow incrcial controls.
Since 2012 the minimum threshold is abolished (all invoices paid with cash or bank transfer will be to be declared, those paid with credit card are automatically!).