Containers in the left wheel well???

Hello,
I removed the wheel arch liners from my Golf to install a new set of rims (which requires registration, etc.).
'I've noticed a container on the left side, near the wheel arch liner; I suspect it's a vacuum reservoir. It's been disconnected and plugged for about a week now, and nothing seems to have changed. Besides my problem with the empty tank (see topic: 'Tank empty...'), which I don't think is related.'
Now, out of curiosity, I wanted to ask what that container is for.
Thank you.

Volkswagen Golf 3 AFN, 153,000 km.
Deeper, wider, faster!! *laugh*

Hello,

The container is the vacuum storage tank.