A closed boundary has the same start and end point. For example when milling a pocket you would have a closed boundary. An open boundary the tool starts at the beginning and stops at the end. For example milling some edges of a block.
John Joyce, Manufacturing Engineer, Senior Aerospace
NX 184.108.40.206 Vericut 8.0.3 - Statements and opinions are mine alone and do not reflect the opinion of my employer or any other member of the human race