The signature certificate required to sign the FatturaPA file must be a certified signature certificate, i.e. issued to an individual on presentation of an identity document. The certified signature certificate can be requested from those on the list of authorised certifiers published on the "Agenzia per l'Italia Digitale" [Agency for Digital Italy] website. The Exchange System accepts the following signature formats:

• CAdES-BES (CMS Advanced Electronic Signatures) with a structure compliant with the public specifications ETSI TS 101 733 V1.7.4, as set out in the appropriate legislation from 1 September 2010,
• XAdES-BES (XML Advanced Electronic Signatures), with a structure compliant with the public specifications ETSI TS 101 903 version 1.4.1, as set out in the appropriate legislation from 1 September 2010. The only method accepted for this type of signature is the “enveloped” method. In addition the XAdES signature must have the Reference element with URI=“” or with URI=“#iddoc” where iddoc is the identity code of the document to be signed: the URI attribute cannot be omitted from the Reference elements.

This question is answered by the so-called 'VAT Decree' (Presidential Decree 633/72, article 21, subsection 1): “For each taxable transaction, the subject who sells the object or who performs the service issues an invoice, also in the form of a note, account, fee or similar, or, it always remaining his/her responsibility, ensures that it is issued on his/her behalf by the intermediary, by the principal or by a third party”.
Therefore, if you use an intermediary for the issue of the invoice (issue and not simple transmission), this intermediary, as the issuing subject, must digitally sign the document, although the responsibility towards the tax authority remains that of the seller/provider.

No, there is no relationship; the former only has the purpose of differentiating the name of the FatturaPA files transmitted to the Exchange System by the same subject, while the second represents the file identity for use by the transmitter.

The progressive number which is part of the name of the FatturaPA file to be transmitted to the Exchange System need not necessarily be strictly progressive, and its composition may vary.

The transmitting subject ('soggetto trasmittente') is the subject who physically transmits the FatturaPA file to the Exchange System (it may be the supplier or a subject appointed by the latter to make the transmission). It must be noted that the 'ProgressivoInvio' field refers to the unique progressive number that the transmitting subject assigns to each file that it forwards to the Exchange System.

the obligatory Electronic Invoicing rules for invoicing toward the PA cannot alter the legislative framework in force regarding tax and/or administrative issues. Some fields, although optional according to the specifications of the “Fattura PA” format can be considered by the single administrations as binding for the payment, on the basis of specific provisions or contractual agreements. In other words, an invoice that is not rejected by the Exchange System could be refused by a single client administration because it does not have all the specific information (which is optional for the Exchange System).