what is difference between MANUAL HVAC blend door actuator and an AUTOMATIC one?

your a/c controls if auto will be most likely digital disp the manual will be dials to control temp settings

If you're replacing the heater core you do not have to replace the blend door actuators unless they are defective. Keep in mind that to replace the heater core in your vehicle the dash board needs to be removed to gain access to the heater core access panel. DO NOT attempt this job unless you feel confident that you can complete the task.