This is real handwavy and potentially wrong, but maybe helpful. Closed is the thing you're _enforcing_. You have to pay your taxes. Open is the policy that you

This is real handwavy and potentially wrong, but maybe helpful. Closed is the thing you're _enforcing_. You have to pay your taxes. Open is the policy that you don't care much about. Cash, check, money order. Maybe others get tacked on later. I know it doesn't sound like much, and it's really a pretty weak tool compared to stuff like contracts. The best example i can dredge up is gui components. I don't care _what_ you draw. I do care that you stick to the small area of the screen set aside for you. The base component _enforces_ no coloring outside the lines. Inside the lines, you can be a textbox, or a button, or a whatever.