One is the compensation water tank and the other is the main water tank. The two water tanks of the car are filled with antifreeze, which is used to cool down when the engine is running. There is a highest line and a lowest line on the compensation water tank. If it is lower than the lowest line, it is necessary to replenish antifreeze, but not higher than the highest line, otherwise the cooling water will overflow when its temperature rises and expands. Highlander has two auxiliary water tanks, because one is the compensation water tank and the other is the main water tank. Advanced models, such as Highlander, a high-power version of BMW-Benz small-displacement turbocharged, will use independent cooling water system to cool the intercooler.