A snap line is a component that is able to generate a point in a line given a point by another component. Usually this is used in components like the Multi User Avatar Anchor Component to determine where to place the anchors upon clicking (click point is used in such case) When provided a point, it finds the closest point on a line and returns that to the component that called it.
The functionality of this component is internal, and cannot be used by ProtoFlux to snap points using in game code.