Levels are level. But what if they weren’t?
Imagine a building on a sloping site. The ground floor is stepped. Conceptually, everything on the ground floor could be on Level 0, no matter what its actual elevation is. Level 0 could just tell you what the elevation of the ground floor is at any point on the site.
I’m currently dealing with a project which has just ten storeys, but over 100 distinct floor levels.
Another example from a 2006 AUGI post: A mile-long building on a slope of 1/32" per foot.