How much before your waistband do you want your last button to sit?
Having buttons below your waistband is considered by many to be an annoyance. They get caught up in your belt, and complicate bathroom breaks.
By default, the last button sits 1cm above your waistband, but this option allows you to move it further upwards for that shirt you are going to wear with those high-waisted trousers.
If you want to have buttons below your waistband, you can. You should enter a negative value here, making sure it's within the length bonus you have available.