Where does EC188 run from and to ?
Generally, a train, run in one country by another country takes the policies of the country it is running in.
So a DB train running in Italy has the rules of Italy. If you get free DB coupons but not Italian then you would have to pay.
IC & EC trains in Italy normally have compulsory reservations. You travel free but a few Euros for the reservation.
Trains from Austria to Italy via the Brenner pass require a supplement of about £15 - even if you have coupons for the entire journey