Requests for technical support from the VASP group should be posted in the VASP-forum.
This file contains the force field information.
It is needed to run calculations using force fields only (ML_ISTART=2).
A knewly created force field with ML_ISTART=0 or ML_ISTART=1 is written to the ML_FFN file. This file has to be copied or moved to ML_FF for a force field only calculation.