MRF method is always applied to certain cellZone (list of volume cells). cellZone can be created using utility topoSet, see setting set in file system/topoSetDict:

actions
(
    {
        name    allcells;
        type    cellSet;
        action  clear;
    }
    {
        name    allcells;
        type    cellSet;
        action  invert;
    }
    {
        name    rotor_MRF_zone;
        type    cellZoneSet;
        action  new;
        source  setToCellZone;
        sourceInfo
        {
            set allcells;
        }
    }
);

This just selects all the cells (now in the region rotor) and creates cellZone called rotor_MRF_zone. Run the utility topoSet:

 topoSet