About Defining Points for Curves
To create a curve you must specify two or more points. Use the 
Curve tool to create curves. Two types of points can be used to define curves:
Curve tool to create curves. Two types of points can be used to define curves:• Free—Points that are unconstrained.
Free points are projected by default onto the currently active datum plane, but their depth can be specified from another view. If you are working in four-view display mode, you can specify the depth in one of the other panes where the green depth line is visible. In single-view display mode, you can rotate the view until you see the green line through the point, and then click anywhere along the green line to specify the depth of the point.
• Constrained—Points that are constrained. The two types of constrained points are soft points and fixed points.
◦ Soft Points
A soft point is partially constrained. It can slide along its parent object. Create a soft point by snapping the point to any curve, edge, quilt or solid surface, scan curve, facet, datum plane, or datum axis. While you create a soft point, the entity you are snapping to is highlighted briefly. Soft points are displayed as circles when they reference other curves and edges. Soft points are displayed as squares when they reference surfaces and datum planes.
 | • When you drag a point to snap, hold the SHIFT key down or click  Style > > .• If a point can be snapped to multiple entities, select the soft point, right-click, choose Pick Soft Point, and click  or  to select the required entity. |
◦ Fixed Points
A fixed point is a fully constrained soft point. It cannot slide on its parent. A fixed point is displayed as a cross hair. There are several ways a soft point becomes a fixed point:
▪ A curve is snapped to a datum point or vertex.
▪ Soft points on free curves become fixed points if the option Lock to Point is used. Lock to Point moves a soft point to the nearest point on its parent curve and snaps it to the point.
When planar curves are snapped to existing entities, the points are fixed because the plane forms an intersection with the other entity.