Separate from Surface is a function to changing inner curves to outer curves.
2D View > Inner Curve > Context Menu > Separate from Surface
- Draw an inner curve inside the surface via 2D menu > Curve.
- Select the inner curves in All / Curves Selection Mode.
- Right-click on the selected inner curve to open the context menu, then select Separate from Surface.
- You can check the inner curve being changed into an outer curve in 2D view.