They are definitions according to IEC 158-1 for contactor utilisation
AC1 is all types of ac load with a power factor not less than 0.95
AC3 applies to squirell cage motors with breaking during normal running. On starting the contactor makes the starting current which is between 5 and 7 times FLC and on opening breaks the motor rated current
just to complete the set
AC2 applies to starting, plugging & inching of slip ring motors
AC4 covers starting plugging and inching of squirrel cage motors.
Hope this helps
John
