Wireless Unit / D-Fly (ANT, deprecated)
This older generation Wireless Unit or D-Fly allows your Di2 system to transmit information through ANT Private network to compatible cycle computers. If you have this installed you will be able to see the Di2 battery level and gear position on your bike computer. Note that it does not support BluetoothLE, so you cannot use the mobile app with this.
The current models are the EW-WU111 / EW-WU101 Wireless Units. The former is an inline two-port junction while the latter has the same shape as the SM-EWW01 and is mounted on the seat stay - connected to the rear derailleur.
This is the only D-Fly that will work with the older SM-BTR2 battery or SM-BMR1 / SM-BMR2 battery mounts.