This bit is active when the 6K is commanding motion. During a normal move commanded with A, V, D, and GO, the motor is "moving" from the time it is given a GO to the time that it stops. This is the same for more complex motion programmed and started with the PCOMP and PRUN commands.
The Following functionality is slightly different. An axis that is defined as a Slave will show that it is "moving" from the time that Following is enabled on that axis and told to start Following with the GO command all the way to when Following is stopped on that axis. When the Master is not moving, even if the Master's bit shows "not moving," the Slave is still technically Following until it is told to stop.