According to the Mstn help section on the fixed 3d Constraint, you can fix
So, applying the Fix Constraint(s) to the Slab, I expected not to be able to move the Slab at all. And when applying the Fix constraint to the Shape, I expected it to be glued to the horizontal plane.
Using the Manipulate 3d Constraints tool seems to bring out the expected behaviour. Not very useful or consistent compared to the other Constraints if this is WAD.
Using OBD 9.2 (Mstn 16.3)