Attenuated forcefields are a downsized version of the Forcefield generators, used to protect the smaller Scrin "vehicles". When upgraded from the Nerve Center, it grants their Gun Walker, Shard Walker and Seeker an extra layer of protection. While active, the forcefield can absorb an EMP blast before collapsing, allowing these units to remain active.
Despite having similar names and effects, the Scrin and Reaper-17 have separate Attenuated Forcefield upgrades. Researching forcefields with a Reaper-17 Nerve Center will not allow units from other Scrin factions to obtain the upgrade. To have the upgrade on both Reaper-17 and Scrin units, the upgrade must be researched twice. Traveler-59 does not have access to this upgrade, because it does not carry over from other Scrin factions.