Part Modeling > Edit Features > Split Surface > About the Split Surface Region Feature
  
About the Split Surface Region Feature
Use the Split Surface Region feature to split a region of a surface away from the original surface so you can work on the split region separately from the rest of the surface. The surface you split can be solid geometry or a quilt.
To create a Split Surface Region feature, you select contours of a single surface. The selected contours are split away from the original surface, and an identical copy of these contours is created to form the Split Surface Region feature. After the split, there are two surfaces: the original surface containing the contours that were not selected, and the Split Surface Region feature containing a copy of the selected contours.