Hi,
A Rect is an abstract representaion of a rectangle, you make up what ever you want out of the values. It doesn't belong to the XY plane, rather it uses a common naming convention to ring bells as you work with it. but basically, you can store in the "y" your z position, that's completly fine.
So for example, you have your vector (10,0,20) which is you start point in your game, inject 10 in rect.x and 20 in rect.z, that's completly fine.
Bye,
Jean