Sorry for the late reply! Each stepper driver has 3 legs named “MS1”, “MS2” and “MS3”. Each of those legs is conected to a pin in the ControlPCB. Jumpers connect “MS” legs to “HIGH” level signal (5V). So, having the 3 jumpers connected means to have a 1/16th configuration (this image ilustrates all the possible configurations). My recommendation is to connect all pins to get a 1/16th configuration, but if you want a different one you have to put the jumpers and modify grbl according to the new configuration.
Hope it helps!Angel LM