I was told during my AS 332L2 conversion at Helisim that TDP is a fixed point which is predetermined based on ambient conditions so we have a fixed airspeed and ht above which we are to continue. For CDP, a good example would be take off from the rig where the CDP is the point of rotation. This point is not fixed by the numbers but more by pilot's feel ..and maybe not the most accurate point as it can vary slightly between pilots. I wonder if most of other pilots agree to this.