The Symmetry node adds a boundary condition that represents symmetry in the geometry and in the loads. A symmetry condition is free in the plane and fixed in the out-of-plane direction.
The Symmetry node is only available with some COMSOL products (see
http://www.comsol.com/products/specifications/).
The value No displacement gives a standard symmetry condition.
Select Free Displacement to allow the symmetry plane to translate in the normal direction. The displacement is determined by the criterion that there is no resulting reaction force in the normal direction.
Select Prescribed force to prescribe the total reaction force acting on the direction normal to the symmetry plane. Enter the
Normal force Fn. The force is defined as positive when acting along the outward normal of the symmetry plane. Setting the prescribed force to zero gives the same effect as using
Free Displacement.
Select Prescribed displacement to prescribe the displacement in the direction normal to the symmetry plane. Enter the
Normal displacement un0. Setting the prescribed displacement to zero gives the same effect as using
No displacement.
) and select Advanced Physics Options in the Show More Options dialog box. 
