Surface-surface trimming has two types of surfaces, trimming surfaces and trimmed surfaces. The trimming surface is the surface that does the cutting. Think of it as a pair of scissors or a knife. The trimmed surfaces are the surfaces that are cut. Think of them as the paper. You can have only one trimming surface, but you may have more than one trimmed surfaces.
The first step of surface-surface trimming is calculating the intersection of the trimming surface and the trimmed surfaces. This step is only visible if you are previewing your calculation before performing it. This curve is then used to trim away a portion of the trimmed surfaces. If these surfaces are the original surfaces, the blue curve is the intersection curve between the two surfaces.
These surfaces are the result of using the vertical surface as the trimming surface to cut the horizontal surface. If you select the Trim this surface also check box, the trimming surface is also trimmed using the intersection curve as in this image:
