Line Snap
While you create a line in a 3D sketch, you can snap the line to geometry
that already exists in the part, such as model surfaces or vertices, and
sketch points.
Snap is not enabled if you are sketching in one
of the major coordinate directions.
If you are sketching on a plane, and the SolidWorks
software infers a snap to a non-planar point, a temporary 3D graphics
box is displayed to indicate an off-planar snap.