I do not know the precision you need depending on your application, but the precision of the MoveBy command depends on the precision of the position estimation of the anafi.
The position estimation uses mainly GPS and vertical camera, when available ; it also integrates acceleration measures from the IMU, but as you may know if only the IMU is available the position estimate drifts because of bias integration and quickly becomes unusable.
So, when only GPS is available ( if flying at far from the ground for instance or if the vertical camera can’t see), the estimate precision is that of the GPS, so a few meters (depending on the quality of the signal received and the number of available satellites).
If the position can be estimated through vision sensor, it is more accurate, around 10cm or so.
I hope that answers your question !