Click Project 3D Canonical Silhouette Edges in the Operation toolbar (3D Geometry sub-toolbar).
The canonical silhouette edges are created onto the sketch plane.
These silhouette edges are yellow if they are associative with the 3D.
You cannot move or modify them but you can delete one of them which means
deleting one trace independently from the other.
Note:
You can only create a silhouette edge from a canonical surface whose axis
is parallel to the sketch plane.