It,s the oil catch can, the fumes from the rocker cover pass through it and oil vapour condenses in can instead of passing through throttle body to be burnt.
Keeps the throttle body clean and coke free, just drain it every so often.
It's from the 6 cyl As200 altezza, same engine as the is200, Only difference is the As200 is set to run on 100ron fuel and is rated at 160hp, due to different ecu map.