Question about iRobot Roomba Combo j5+

the app's advanced installation guide for the mop function talks about a plastic tray (water protector) for the station that is supposed to be in the box - but i haven't found this. only the PET transport protector that the roomba was lying on in the box. it doesn't look like the one in the description, nor does it fit in the station, and it doesn't look like the roomba can ride on it when it returns to the station. so i'm wondering what this mysterious plastic thing is?