[vtkusers] SetDataSpacing

[a.dek at libero.it]

Hello! My name is Antonella
I am new in vtk so excuseme if my question is stupid.
I will try to visualize a medical data set.
I would like to know what SetDataSpacing()'s values rappresent.
I suppose that in z axis it is the distance between slices, but in X 
and Y axis?
I have another question:which is the unity of measure for this 
parameters ? (pixel,millimetre, etc.)
thanks in advance, I hope to hear from you very soon
Antonella