It is confusing, between different vans, options, ... and lack of clarity or wrong imagery from MB at times.
That little battery depicted is the typical MB backup battery, that will / should allow the vehicle to be powered up and be shifted into neutral so it can be towed, when the main ( starter ) battery is depleted too much. I say should, because I had a personal experience this summer with our SUV, where she had zip nada??
There can be battery alerts about battery replacement where people replace the main battery, still have the error as the tiny one is toast. MB hopefully have gotten better about specifying which in recent models.
Oh, and the tiny battery can be in wildly different locations depending on the car model, like: either side of the dash, passenger seat, trunk.