Hi evil_cz.
I give you an example to clarify.
By example I am a free professional so I have a VAT (IVA) number. However everyone here also have a Fiscal Code Number (wich is calculated from name, surname, date and place of birth).
When I make an invoice, I need to expose both this numbers. Or if I buy something and I need an ivoice for tax purposes, on this invoice I need both my VAT number and Fiscal Code

I hope it helped