Many misunderstand the purpose of the AMC644USB board and what exactly it does.
First of all, the VFD inverters are just speed drivers for the AC motors meaning that they only set the speed and the direction of the motor. They are not intelligent in any way to be able to determine the position of the motor or place the motor to curtain position. Some inverters have feedback from the motor just to verify the correct speed. Even if you connected them to a computer through a RS485 interface the only thing you can do is to adjust their speed.
This is where AMC644USB comes into to fill in that missing link of motion control. The AMC644USB receives the motion data from the computer (positions) in high speeds of 1ms intervals, then creates the proper voltages and signals to set the correct speed and direction needed from the inverters and then reads the position of the motors using the potentiometers or any other encoders attached on the motors as feedback to precisely stop each motor to the desired position. These positioning calculations are performed 5000 times per second that gives fine positioning resolution and very fast reaction times.