You can go anywhere you like, using the participating railways companies. On a travel day, you can use as many trains as you like. There is no limitation other than the time needed and you can only travel in your country of residence on 2 of your travel days.

Example from la bella Italia:

(for the turistice that come to yours); can use ANY TrenTalia or TreNOrd treni. canNOT use oltre ferrovie locale, like in Umbria or Puglia (as non Tr-It).

Can go Napoli-Salerno on Tr-It (choose between frecce=pay a lot extra, or regionale) NOT on the other ferrovie.

MOst common hurdles that this forum is full of: ALL treni in Espana  MUST be reserved and cannot be done online, same for TGV in FRancia (can do online, but small special quota only), VERY multo high supplemento to use TGV-treni over the border to FRancia.