Edit Surfaces

Each surface contains one or more faces. It is usually preferable to combine multiple faces into one surface entity before you use the meshing tools.

This allows them to be meshed at the same time.
Use the Surface Edit panel to modify surface geometry when it is necessary to make changes before you generate a mesh.
For example, to trim a surface with a line, use the trim with lines subpanel of the Surface Edit panel. You must select the surface and the line and specify a direction vector. The surface is trimmed by sweeping the line along the vector and intersecting the surface with the sweep. If the sweep does not intersect the surface, the surface is not trimmed.

Figure 1. . A circle and a surface (represented with surface lines) before trimming.

Figure 2. . After the circle is used to trim the surface, two new surfaces are created (shown highlighted) and the original surface is trimmed.
To trim one surface with another, use the trim with surfs/planes subpanel.

Figure 3. . Two surfaces before trimming.

Figure 4. . The smaller surface is split into two surfaces after it is trimmed with the larger surface.