From Demauro:
pro|ba|tò|rio
agg.
TS dir.
1 che ha valore di prova: elemento p.
2 che attiene alle prove: documento p.
Note in both cases this refers to "proof". The first link provides a list of the "documentazione probatoria da allegare alla domanda di accreditamento"supporting documentation to be attached to the refund (credit) application
Broadly speaking, a certificazione is a document that states/affirms/confirms a characteristic of the item(s) it refers to, it does not have to be something that is notarized in the american sense of the word.
Here we'd say the required documentation (which supports the application) i.e. w2' that must be attached to tax returns would be a "certificazione probatoria", they constitute proof of how much one made for the year.

In this case, the company " has explained how collecting the required support documentation prior to date the benefit was only mandatory under the original version of art. 27 bis"

In other words..it may have been a condition once upon a time but, as the company already explained, it ' s no longer the case.


cer|ti|fi|ca|zió|ne
s.f.
CO
1 il certificare
2a certificato (accorc. certifica)
2b autentica: c. di un documento (accorc. certifica)
cer|ti|fi|cà|re
v.tr. (io certìfico)
1 CO TS burocr., attestare, dichiarare formalmente, spec. con un documento scritto e sotto propria responsabilità: certifico che il sig. Rossi è idoneo a frequentare la piscina
2 BU assicurare, rendere certo di qcs.