Could you please explain the "vouvoiement" rule?

One aspect of the French language which can be confusing at first to non-native and especially English speakers is the formal use of the pronoun « vous » and its subsequent verb form. The « vous » form usually corresponds to the 2nd person plural but is also used instead of the 2nd person singular « tu » to address more formally an individual in specific contexts as a form of respect. « Vous » is generally employed to address an elderly person (or a significantly older adult if you are a child/young adult), someone you don’t know/have just met, a hierarchic superior or figure of authority. « Tu » is used in more informal contexts, to address someone who you consider your equal in rank or age, someone you have a close relationship with (family, friends).Examples:If talking to your boss : « Bonjour, comment allez-vous? Avez-vous bien reçu le document que je vous ai envoyé ce week-end? »When addressing your school comrade: « Salut, comment vas-tu? As-tu bien reçu le travail que je t’ai envoyé ce weekend? »N.B. It is quite common to use « vous » at the beginning of your acquaintance with someone and to later slip into the more familiar « tu » once you have gotten to know each other, unless you are addressing a source of authority.Example: using « vous » when you first meet your friends’/significant other’s parents, then over time shifting to « tu »