[meheleventyone]

It’s just convention which axis represents up. Typically you don’t see the axis labels rearranged like in the OP but instead coordinate systems described as y-up or z-up. Similarly left and right handed.

[yreg]

Well, it's about the order. In vanilla Minecraft client you see height as the second coordinate out of the three.

[meheleventyone]

Right because vanilla Minecraft uses y-up, so the second value ‘y’ represents height.