Size of the border around the edge of the group. 

Typically you don't want the group to go right to the edge of the frame, you'll want a border to allow for mounting, or to allow for the finished photo to be a different aspect ratio to the camera. Leaving a border around the edge also gives you a margin of error if your measurements aren't quite right, or you can't get the camera to exactly the right height. 

Recommended values for the border in 20% or 25%. The default value is 25%.

You can change the default border in Settings (Pro Version only).