You can create one or more curves in a sketch by projecting an edge, loop, face, curve, or external sketch contour, set of edges, or set of sketch curves onto the sketch plane.
To convert an entity:
With a sketch active, click a model edge, loop, face, curve, external sketch contour, set of edges, or set of curves.
Click Convert Entities on the Sketch Tools toolbar, or click Tools, Sketch Tools, Convert Entities.
The following relations are created automatically:
An On Edge relation is created between the new sketch curve and the entity, causing the curve to be updated if the entity changes.
A Fixed relation is created internally on the endpoints of the sketch entity so the sketch can remain in a "fully defined" state. This internal relation is not displayed when you use Display/Delete Relations. Remove the Fixed relation by dragging the endpoints.