The SC-M9051 System Information Display shows you what gear you're in, has a battery level indicator, charging port, synchronized shift mode display and also doubles as a Junction A - it has 3 e-Tube ports for you to connect components to it. The displays lets you charge the system and also connect to it using Bluetooth or ANT+.
If you're thinking of using this display on your road bike to replace your Junction A (SM-EW90 / EW-RS910) and Wireless Unit - you can! Be aware that you can only have one component with wireless/bluetooth functionality connected. If you connect both this and a Wireless Unit the Di2 system won't function.
Shimano made another System Information Display - the cheaper XT series M8050 Display. It is identical to its more expensive XTR sibling, except for the casing.