The idea is to have an option to select the nearest valid unit to the current one instead of selecting units in the order they were deployed. The latter case is less useful in most scenarios because soldiers will spread over the map based on their role and equipment, not in the order they walked off of the skyranger. So simply clicking "next unit" will often bounce the camera all over the map as it selects soldiers in seemingly random order.
How I would implement this is a shift-modifier. So shift clicking the Select Next or Don't Reselect buttons would pick the nearest valid soldier to the current one. This would also apply to the keyboard shortcuts, shift-Tab for Select Next and shift-\ for Don't Reselect..
