Where you remain for 15 minutes or more with 20 or more people.
Where you remain long enough for one person who is carrying to breathe enough viral load into your airspace.
I remain stunned by the people who think they will not endanger themselves and others. I wish the OP diner in the restaurant would explain why they think this is safe? Let's assume he is a programmer or some kind of educated person who thinks basic science, math, etc works. Why would they think it's safe to go to a restaurant where it is an enclosed space.