The Shimano SC-MT800 XT Di2 Digital Display Unit serves several vital functions in any Mountain Di2 groupset. It is a valuable display for information like battery level and gear information. It also acts as both the "A-Junction" to connect your shift controls to the battery and derailleurs, but also is the external charging port to charge the internal battery for the system. Using the control screen you can also use shift mode features like synchro shift and select different rider profiles and gearing setups within the system. The SC-MT800 is also compatible with the D-Fly transmitter for communicating with devices like cameras and Garmins, but is also compatible with smartphones like iphone and android devices to setup and change settings using the e-tube app.