The Vector Boundary form allows you to create boundaries around selected vectors.
When this is checked ✓ the created boundary is offset outwards by the distance specified.
When this is checked ✓ the created boundary is the result of stretching a rubber band around the currently selected vectors.
The images below demonstrate the difference between the two types of boundary that the form creates. The picture on the left illustrates the standard offset output and the one on the right shows the result when Rubber band boundary option is checked ✓.