System Information Display - Junction A
The SC-MT800 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 Di2 components to it. The displays lets you charge the system and also connect to it using Bluetooth and ANT+. It is usually installed on mountain bikes.
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! Note that you can only have one component with wireless/bluetooth functionality plugged in. If you connect both this and a Wireless Unit the Di2 system won't function.
Current generation battery or mount required
Just like the SC-M9051 display, this SC-MT800 requires a modern battery (BT-DN110) or battery mount (BM-DN100). The display will not work with the older batteries.
Synchronized shifting on mountain bikes
If you want to use synchronized shifting on your mountain bike the usual requirements apply - you need either a BT-DN110 battery or BM-DN100 battery mount.
You will also need a System Information Display. It is not clear why exactly you need one, but I've asked Shimano and they have confirmed that a display is required for synchro shift on mountain bikes.
XTR display vs XT display
In case you are wondering what the difference is between these two displays - there is none. The casing is different, but the functionality is exactly the same.
Pairing the SC-MT800 - when the 'C' doesn't show
If you've been trying to pair one of these to a mobile phone you may have run into the issue of the 'C' (connection mode) not showing. Some of these units do this, depending on the firmware version. This generally isn't a problem, because they do actually go into connection mode. They just don't show it.
I have written more on this in the SC-MT800 Installation guide.