Where: A kitchen in a midsized apartment on the outskirts of central Petersburg. Four families live in the apartment.
What: As noted in the clip "Washroom" (Tour 2), hot water comes to the washroom from a gas heater located in the kitchen. This is the case in many apartments. If hot water is being used at the kitchen sink, then anyone using the bathtub will not have enough.
The terse sign "bathtub in use" alongside the water heater is meant to be hung over the faucet when someone is washing in the bathroom. The sign is meant to stop people from using the faucet in the kitchen. The piece of cardboard used for the sign turns out, if you look on the other side, to be the label from a pair of jeans.
Sometimes the same purpose is fulfilled by wrapping a rag around the faucet or putting a piece of paper on top of it.