The tail light I believe. The newer bike powers it from the main battery if I remember right. That and the freewheel. For '19, they made a change that had been a popular mod earlier, which allowed you to stay up with the pedals to speeds up to about 20mph vs. the '18 and older bikes where most of us were all done by 15mph or so. Pretty easy mod. that I would recommend.
You can get that DNP freewheel with a 28 or 34 tooth low gear now. Your choice of whether or not you want a stump pulling granny gear available. For me, it was the 11 tooth high gear that I was after. That's the one that will let you stay up with the pedals.
The OEM '18 freewheel was not a DNP, and the smaller gear had more teeth - making most of us unable to stay up with the pedals at even modest speeds.
Tail light was powered with a watch battery I think. I've never used mine!
My '18 City has a Shimano Acera, which continues to serve my purposes well. If I know Rad, betting the Rover will be same.