Compushift is a stand alone gearbox ECU made by HGM Electronics. The current version is a variant called Compushift Sport/Pro (aka Compushift 3). The difference between Sport and Pro is number of gears supported, with the Sport only supporting 4 speed transmissions, and the Pro supporting more gears.
This version is configured via Bluetooth from your smartphone, instead of having to deal with up/down/left/right/enter to configure it. It’s pretty snazzy, and advanced for this sort of thing. It requires very little inputs in order to make it work: 12V, Ground, Throttle Position Sensor and the gear shifter sensor. You can feed more information to it in order to make it better, either via CANBUS or wiring in extra sensors such as MAP sensors.
I hear you say: ‘Why didn’t you use the Disco 2 Autobox ECU?’ There’s 2 reasons. The first one is that the Bosch unit that comes in the Disco 2 requires connections from the BCM and other connections from engine ECU that Defender TD5 doesn’t have. It could be made to work but it’s a lot of faffing around. the second is that the Defender and Disco sit somewhat differently, and the benefit of the Compushift (or any other aftermarket TCM) is that you can tweak the settings to make it shift correctly.