Id say it would be the scale required (20nm)to be an on chip solution to replace the sram and Nvm in 1( nvm currently sits off chip i believe due to scaling limits.) there is an excellent diagram on the cousins website which illustrates this perfectly. its from a media article in regards to scaling edge Ai with Reram and is pretty much what 4ds is aiming for ,
search electronic specifier if you wish or go to the media coverage section at WBT its worth it.
Im sure someone more technically may be able to expand further .
