This environment variable is used to simulate a more correct neutron production rate by the photonuclear effect in the region of the giant dipole resonance. The envvar is not documented. See https://geant4-forum.web.cern.ch/t/understimating-neutron-flux/999/4 and https://bugzilla-geant4.kek.jp/show_bug.cgi?id=1680
This is now documented in the 10.7 documents.