Automatic start. A separate relay on the starter activation line. Can only be implemented with certain engine control unit versions.

https://bmwteka.com/wds/en/e53/139b5a53

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E39 530dA -02 M-Sport Messing metallic
E53 X5 3.0dA -06 Sport Stratus grey
E70 X5 40d -12 M-Sport Space grey