What is the difference between using the border setting with SmartEdge and without it?

Any motion in the border is ignored.  This is used to exclude areas where regular motion occurs (like the motion of a fan for example) that is not relevant to the movement you want to monitor.

If SmartEdge is on the border size is fixed.  If there is motion in the border before motion in the center, the alarm is suppressed.  If there is motion in the center first, the alarm will sound as usual.  This feature helps prevent false alarms when a person is going to sleep or when a caregiver enters to check.