Symmetry Line
The Symm Line constraint identifies a line of symmetry that can be used by AUTO to assign mirror constraints. If you want to assign mirror constraints by hand, you should use ASSIGN Mirror instead. You assign Symm Line constraints to linear elements, which are then labeled with the symmetry line icon,
Constraint Icons.
To assign Symm Line constraints:
1. Select ASSIGN on MENU 2.
2. Select type Symm Line.
3. Pick the linear element to be constrained.
4. Press END or assign the next constraint.