K is correct. And so is ambershee.
Unreal was around long before Maya, but basic math was around long before Unreal… and as K stated, Math defines X, Y & Z accordingly.
I’m not sure where some of this ‘other math’ comes from. Basic geometry in grade school teaches you that Z is the "up’ or vertical plane.
The only time Y will be the ‘up’ is in a 2-D axis plane such as a sheet of graph paper. Rise over Run.