Trains in Italy- all need ticket validation?

Tickets purchased in advance- online or app- do not need validation
Regional tickets purchased at station/kiosk do need validation

https://www.seat61.com/train-travel-in-italy.htm#travel-tips

I'm not sure I get what you mean with "regional bus", but the rule is: No train ticket purchased online must be time-stamped.

Only Regional trains tickets and only when purchased:

• at the counter
• at ticket machines
• at travel agencies

must be time-stamped before getting on the train.

It's not a big benefit. The Regional train tickets that must be time-stamped are more flexible.

IC and Freccia tickets do not need to be validated, since they include seat reservations and the ticket is good only for that train on that day. Regional tickets bought at a ticket window or out of a vending machine need to be validated because they are open tickets, good for any regional train on that route any day or time. So you validate it to show you are using it for the train you are riding on and cannot reuse it on another day.

Just to be clear. When you validate a Regional train ticket, you start a clock so that ticket is only valid for four hours (I think). Someone will correct me if the time is wrong. So don't validate too early prior to boarding the train. The reason you don't validate an on-line Regional ticket purchase is that when you purchase the ticket you state a starting time for the ticket so that ticket is only valid after your stated start time. Change your mind and the ticket could be no good. See Dario's last sentence.

No train ticket purchased online must be time-stamped.

Think Dario meant to say
No train ticket purchased online must be validated.

That is what validation is --- a Time stamp. Starts the clock ticking on that ticket till it expires.