[vtkusers] vtkMatrix4x4 : Position in the 3 first line of the last column

[Aurélie Larcher]

Hello,
I have a question about the vtkMatrix4x4 :
What represents the position of the matrix 4x4 in the three first line 
of the last column ?
I believed that is the position of the origin of my object in its own 
coordinates : when I translate my object, I see the positions moves.
But when I rotate an object from his center (without translation), 
strangely I see also the positions moves.
Is somebody can explain what are these positions ?
I really appreciate your help
Thanks,