Italian to English Law/Patents Law: Taxation & Customs criminal law on tax offenses

This is from an overview of Italian law on fraud and tax evasion. I've found a lot of reference to this concept of "irripetibilita'" in the context of criminal trials, but I'm not sure that it simply refers to unique or one of a kind documents. Can anyone explain?
Here is the context:
Non tutti gli atti raccolti in sede di indagini preliminari vengono trasmessi ai giudici del dibattimento, ma soltanto gli atti irripetibili e pochi altri.