It’s really not a thing you “use”, it’s just a station near the buffet where the hostess will show you a tablet. Same with the dining room, the servers will bring the tablet over for you to see your options (generally in the MDR you’ll order the night before for the next day’s meals). The tablet lets them select different allergies or preferences to filter everything. I wish it was available as a part of the Hub app or something, it would be nice to be able to see things yourself.
As others have said, I’ve found it kind of hit or miss whether someone is at the station but you can always tell someone you need help with food allergies and they should send someone over.
Basic allergens like gluten, dairy, etc are labeled on the buffet during lunch (for some reason breakfast isn’t labeled with allergens, and I don’t usually eat there for dinner so I’m not sure they’re labeled then), but if you have wider restrictions you will definitely need to talk to someone at the station. I’ll warn you that the person at the station isn’t necessarily super knowledgeable, they’re usually just using the filter and won’t be able to give you any specific info.