This geometrical operation allows to perform a symmetrical copy of your mesh or some of its elements.
To apply symmetry to your mesh:
From the Modification choose Transformation and from its sub-menu select the Symmetry item. The following dialog box shall appear:
This operation has three options, you can symmetrically copy your mesh or some of its elements specifying:
one point
one axis (point and vector)
one plane (point and normal)
Create a copy radio button allows to keep the initial elements or mesh.