Designing z-order terminology in 2d space

It's quite ironic to think why z-order exists for a 2-dimensional space. I mean theoretically it doesn't, but if you look at web technologies and how svg works in 2d space, it creates that illusion to people.

My problem was that how I am going to advocate general audience, who uses whiteboard, by designing correct terminology for them as well as tackling it in a web app. I hope you had a good read here.