Forcefield Generators are defensive devices that generate a tangible plasma shield around certain Scrin units, when upgraded from the Technology Assembler. This upgrade is not available for the Traveler-59 sub-faction, which values mobility over durability.
Upon completion, it is equipped to Devastator Warships, Planetary Assault Carriers, Annihilator and Reaper Tripods, granting them an extra layer of protection. An active forcefield will absorb the shock of an EMP before collapsing, potentially saving these units -- especially the airborne ones -- from certain destruction. Similar shielding was used by the baseline Scrin faction and the Reaper-17 cult to protect smaller units as well.
Since Reaper-17 doesn't employ Devastators nor Planetary Assault Carriers, the cost is reduced to half.