Coolant Recovery Tank
WARNING: This page does not describe the selected car, but rather 41 other vehicles, including the 2003 BMW M5, 2003 BMW M3, 2003 BMW 540i, 2003 BMW 530i, and 2003 BMW 525i. However, it is still accessible from the selected car via links, so may be relevant.
The engine cooling system is equipped with a reservoir to maintain a constant supply of coolant in the system. When the coolant is hot, it expands into the reservoir; when it cools, it contracts and flows from the reservoir into the radiator. The surge tank is transparent, so that the fluid level can be checked periodically, to ensure that the coolant level is adequate.
The level of coolant in the reservoir should be above the bottom line ("Kalt") when the engine is cool and below the top line ("Heisse") when the engine is at operating temperature.