The GT1R R35 Transmission Overflow Kit is designed to allow you to be able to over fill the transmission with fluid to ensure no air is sucked up by the transmission oil pump under high G's. Without doing so, the pump will suck up air and cause transmission pressure to drop enough to lose clutch pressure and/or not shift. If you over fill without a catch can the trans will spill oil on to the track at high G's.
T1 recommends 3.5 quarts over full with this overflow kit. The overflow can is to be installed in the drivers side (LHD car) rear wheel well.