0 will represent the starting point or the reference point. For example, #9 above says "11 feet below sea level" is -11. So, sea level would be 0. For number #11, it says "15 seconds before rocket liftoff" is -15. So, rocket liftoff would be 0. It does not depend on the integer, it depends on the situation, which is unfortunately cutoff in your picture for the questions you're asking about! Anyway, hope that helps!
