For Inner Sphere . AMS , Laser AMS , Small Pulse Lasers , and Machine Guns all can be point defense systems . AMS I think Only can act as point defense systems. The work by reducing the incoming missile volley's damage value by the point defense damage value . For instance someone in a dog fight gets behind your Slayer-16 B with a small pulse laser set on point defense . It gets hit by an LRM/10 that would have done 6 points of damage to say the Aft armor which has 50 points of Armor and therefore a damage threshold of 5 so it would have been a possible critical result. Instead the volley's damage gets reduced by the damage value of the small pulse laser so the 6 point hit does 3 points instead which is below that 5 threshold number and does not generate a possible critical . I believe their is a fighter armed with 5 laser AMS that is used to escort drop ships that reduces any volley of missiles that passes through its space to hit the drop ship .