An adjective describing the atmosphere at a restaurant, bar, or nightclub that is a mix of classy and casual. Usually nicer than a local dive or a "bar and grill," but not as pretentious and luxurious as popular dance clubs that have a $20 cover charge and serve $12 cocktails.
Classual establishments usually offer a variety of vibes, including relaxing lounge areas where people can talk and drink, small dancefloors for the Shakira in all of us, and scenic patios or balconies with nice views of a city/beach/landscape.
Bitch: "Have you been to that new place downtown on 5th?"
Punkin: "Yeah, it's very nice. But also pretty mellow."
Bitch: "I don't really want to get dressed up, but I don't want to go to a place where we have to be around a bunch of asshole frat guys either."
Punkin: "Let's go there then. It's pretty classual."