You can trim and create holes in surfaces by defining a trim/hole boundary. You can think of these boundaries as cookie. cutters applied to the surface. For trims the inside of the cookie. cutter is kept, for a hole the outside of the cookie. cutter is kept.
To create a trim or hole on a surface:
• Select the surface you want to trim or add a hole to.
•
Choose "Trim Surface" tool 
or
"Create hole in surface" tool 
.
The selected surface becomes highlighted in the alternate highlight color.
• Select objects to define the outer boundary of the trim or hole.
The objects selected will highlight and if all selected objects display on a photo the boundary outline will be drawn on any open photos. This is similar to creating a Surface Path.
• When you are finished you can either click back on the first object in the trim/hole, or you can select "Finish Trim" from the right-click.
When the boundary of the trim/hole is 'dropped' on the surface the entire boundary must fall on the surface if it does not the trim/hole will not be applied. Sometimes it is useful to extend a surface before trimming it. Trims can be enabled/disabled and deleted using the Trims & holes setting on Surface Properties.
Note 1: surface draw points, lines and curves can be used as the defining objects in a trim. This provides a simple way to create objects to define a trim or hole.
Note 2: you can only apply trims and holes to NURBS surfaces. This means you can not use Path surfaces in polygon form or Silhouettes.