You must have two surfaces in the model to use Surface/Surface trimming.
Set whether to Create a new surface or Modify the existing surface.
If you’re creating a new surface, name the new surface for future reference.
Click Pick and select a trimming surface, or select the surface from the dropdown list box. When you pick the surface with the mouse, pick the surface in an area you want to keep.
Decide whether to set the Trim this surface also check box. If so, then set the Side kept field to choose the correct side.
Click Pick to select the other surface, or select the surface from the dropdown list box and click Add to add it to the list. You can add more surfaces to the list the same way and trim multiple surfaces all at once.
If the side you selected the surface from isn’t the side you want to keep, then set the Side kept field for the surfaces in the trimmed list. The options for Side kept are:
Picked side will keep the portion of the trimmed surfaces where you picked with the mouse.
Normal will keep the side of the trimmed surfaces that are on the opposite side as the normal of the trimming surface.
Reverse will keep the side of the trimmed surfaces that are on the same side as the normal of the trimming surface.
Set the Tolerance for the operation. This value specifies the distance between calculations along the intersection area. Set the tolerance lower if the intersection curve appears too coarse.
Click Preview to check your results against your expectations. You may need to work with the Side kept fields and multiple Previews to get the final surface you expect.
Click Finish or OK to complete the operation.