Snapping boundaries constrains the control points on the boundary of one surface to the matching control points on the boundary of another. This is useful when you are aligning points on boundaries before assembling a surface mesh.
Choose Create 
Surf Mesh Snap Boundary from the Model toolbar.
If you didn't select both boundaries in step 1, you can pick the remaining ones now.
The points of the first boundary are aligned to the points of the second boundary and the Snap Boundary property editor opens.
Adjust the parameters as desired:
Subsurface specifies the index of the subsurface that "owns" the boundary if one or other of the surfaces you selected was already a complex surface mesh.
Boundary specifies which boundary to snap on the corresponding surface.
Offset is used when the two boundaries do not have the same number of points. For more information, see the next section Snapping Boundaries with Different Numbers of Points.
The Snap Boundary operator is persistent — any time you move the second surface, the boundary points of the first one follow it.
Snapping Boundaries with Different Numbers of Points
If two boundaries don't have the same number of points, you can still snap them. In addition, you can specify an offset to determine which points get snapped together.
To snap boundaries with different numbers of points
First select the boundary with fewer points, then add the other boundary to the selection.
Choose Create Surf Mesh Snap Boundary from the Model toolbar.
If you didn't select both boundaries in step 1, you can pick the remaining ones now. Remember to pick the one with fewer points first.
The points of the first boundary are aligned to the points of the second boundary and the Snap Boundary property editor opens.
In the Snap Boundary property editor, adjust the Offset slider to control which points get snapped to which. Adjust the other parameters as desired.
