About Creating Curves from a Surface
You can create a free curve, a COS, or an isoline curve from an isoparametric line of a surface using the 
Curve from Surface tool.
Curve from Surface tool.• Free curve—Approximates an isoline during its creation. You can freely edit this 3D curve using the 
Curve Edit command. A free curve does not keep the surface from which it was created as a reference or its isoline property. A free curve can be used as an internal curve for further modification of surface shape.
Curve Edit command. A free curve does not keep the surface from which it was created as a reference or its isoline property. A free curve can be used as an internal curve for further modification of surface shape.• Curve on surface (COS)—Keeps the surface from which it was created as a reference, but it does not keep its isoline property. You can edit a COS using the Curve Edit command.
Curve Edit command.• Isoline curve—Connects points of the same value. It remains isoline in relation to the surface even after regeneration. This characteristic benefits the connection of the surface to other surfaces.
If you apply Curve from Surface to a trimmed surface, the resulting curve from surface is limited to the edge of the trimmed surface. You can edit this curve using the Curve from Surface command.
Curve from Surface to a trimmed surface, the resulting curve from surface is limited to the edge of the trimmed surface. You can edit this curve using the Curve from Surface command.