"They're not physical, you can't hold them or possess them, but they're very valuable and powerful...and they'll make mincemeat out of you if you're not careful enough"
Non-Fightable Tanks (or NFTs) were a type of unmanned, invisible, intangible vehicles that were known to exist in the CR era.
At least one NFT was known to have been stationed at a Stuckey's on the island of Cof. It is unknown if NFTs were invented for a war in the CR era, or if they were yet another insane piece of R&D technology invented at a neighboring Alpha Complex.