Wien Venezia Supplement misunderstood

Hi, i’m trying to understand the sub sytem of interail, for example for the WIen-Venezia, as you can see on my screen, I need to buy a Supplement.

I’m not sure where i need to buy these. The interail site guide me to the OBB site, but i’m not even sure it’s the train i will take or if it’s the right things to buy.

Can someone guide me in this dark side of eurail ? haha

Thanks !