It is indeed x>y>z (or z>y>x, depending on your point of view)
It will start at X=0, Y=0, go through all the Zs. Then Y=1, and all the Zs again.
So: (xyz order, 3x3x3 array)
If you want to see for yourself, try the following:
+ System: On start of layout
+ Array: For each XYZ element
-> Text: Append newline&?Array.?CurX&?Array.?CurY&?Array.?CurZ