Select Create > Surface > Trim Plane.
The Trim Plane command creates a composite planar surface at the edge of another surface, along a single curve, along a chain of elements, or within a closed curve.
Cap
The Cap command will create a planar surface at the edge of another surface or along a curve. When you click Cap, you will be prompted to "Select a curve or a surface edge to cap." Select a curve such as a circle, an arc, or a spline, or select the edge of a surface.
Chain
Click Chain to create a trim plane along a series of separate but connected elements. A modified Select Chain menu will be displayed.
Single
Create a trim plane along a single curve similar to the Cap command.
Chain
Click to chain a series of separate curves, creating a single curve, and automatically creating a trim plane along its edge.
Chain Option
Click Chain Option to create trim planes along curves or surface edges you will chain with the restrictions you impose using the Chain Options dialog box.
Auto
Click Auto to automatically create trim planes along all contours you select using the Select menu. You will be prompted to "Select contour(s) to be chained, then ‘Done.’"
