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It is an implied thing not a tangible written down thing. Which is common in life. For example you are sitting on a bus with your headphones on, it is understood that I don't sit next to you and try to talk to you, unless something out of the ordinary happens and this is critical. In the grocery store you know that your cart goes on the same side of the aisle as you drive on.

All this stuff adds up and it becomes a basket of little rules. Follow them and things will go smoothly, don't and they will go less smoothly. It doesn't mean that your life is going to be even good it doesn't mean nothing bad will happen to you it doesn't mean everyone who breaks it will be punished. It is not even aspirational it is empirical. It is is how human civilization works.