By default, Unity creates planes normal to the y-axis. So that if looking thru the default camera the x-axis can be thought of as the width of the plane and the z-axis is the plane depth.
If I rotate a plain -90deg about the x-axis, this will make no difference as far as scaling goes: x will still be width and z will be what I’d think of now as height of the plane. First I thought I was misunderstanding the frame of reference for the plane’s transform and so I created an Empty merely as a parent transform; I added the plane with its transform set to identity as a child of the empty and then I rotated the empty about the x-axis by -90. But still the same result: I have to use the z-axis for the plane’s height / width or however we might want to call the other dimension on the plane along with x.
In contrast, this is not the case with a cube object: rotating it about the x-axis by increments of 90deg does alternate between y and z.
So am I missing something or is it just not possible because, say, it was decided that a plane has absolute coordinates?