You can use it on nearly all the lines in Italy. Look at the country specific page on this site. All high speed trains in Italy need compulsory advance reservation see the advice on this site. You cannot use Italo services or small private railways. If you are just visiting the normal tourist cities you will be fine. Enjoy Italy.

There are in IT quite a few minor branch lines where passes are not taken. Most are so far apart and unknown that the usual 1st time tourist will not even realise they are, but some lines in populair touristy areas, like around Vesuvius vulcano (Pompei and the like) and around Etna on Sicilia are also among them-but then there are mostly also lines by TrenItalia close by.

On the hi-speed superfast lines that link all major big cities there is TrenItalia (takes passes, only with RES) and also a private competitor- ITALO_which does NOT take passes. However-the 2 main airport links, Roma+MIlano/Malpensa= these trains are thought of as being only 1st cl.

IF you use the planner of TrenItalia then it should show only trains that take passes.