What is this reservoir for?

[LUw]

After many an hour searching on google, haven't found the 'probably simple' answer to the following question.

What is this reservoir for? The one that I've marked on the photo. (Photo is from other post on the forum)

 

[PHEV07]

After many an hour searching on google, haven't found the 'probably simple' answer to the following question.

What is this reservoir for? The one that I've marked on the photo. (Photo is from other post on the forum)

That plastic Tank contains a special fluid for the Battery Cooling

On a hot day, you may hear something running under the hood especially charging. When this happens open the hood and you should see the blue fluid in the plastic Tank swirling around

 

[LUw]

That plastic Tank contains a special fluid for the Battery Cooling

On a hot day, you may hear something running under the hood especially charging. When this happens open the hood and you should see the blue fluid in the plastic Tank swirling around

Great, thank you for the answer. That clears it up.
Mine is filled until 2cm above the max level. Will take a bit out then just to be safe.