This is a post by Aaron Shaw on PTC site.
With the release of Creo 4, you can now split part of a surface away from the original part in Creo Flexible Modeling Extension (FMX). That way, you can work on the split region separately from the rest of the surface as needed.
In previous releases, the system wouldn’t remove elements if the remaining geometry couldn’t be closed to a solid. The Auto split shape surfaces check box available in Creo 4 now detects and automatically splits the “bleeding” surface—seamlessly removing the shape.