One is for power to the car, lights locks etc, the other is to alternator crank high current stuff, the yellow plug is a small pyro charge to disconnected the high draw in the event of a crash, to prevent fires etc... It goes off at the same time as the two on the inside mounts of your seatbelts, pretensioners if you like, and your airbags, and a couple of unanticipated bodily functions

. Lets face it, you've already been involved in an accident and been yanked back into seat so hard your teeth flew out and are now firmly embedded in your forehead by the airbag going off, no need to add Mr Chrispy to the list. So I hope you never need to test it ...

. This little blighter can cause you to question the Loom of Doom, dont ask me how I know...
See here
https://zroadster.org/threads/z3-battery-cable-route.42375/