Tough I know it is possible to change default Display style for overall structure, I was wondering if it is possible to create default Display style for overall structure whose parts are differently represented? That is to say, if I had slab structure of 4 layers, I wouldn't be interested in seeing two bottom layers (since they dont move in my simulations). In other words, I would like to set geometrical constraint for default display style of type: bottom two layers (defined by z axis) will be presented as line , on the other side, the top two layers are of interest and I would like to see them as Ball and Stick.
This is a variation on Carsten's answer. If you have fixed the bottom layers using Constraints in Materials Studio, you could use a script to query whether the atoms are fixed and set their display style accordingly.