What is levels of formality (0)

What is levels of formality? Search on the Internet leads to such subjects as "Levels of Formality in Language ", "Formality levels of communication" , "Levels of formality in writing".
Proceeding from it, it is possible to draw a conclusion, that levels of formality are almost everywhere.
For a start it is worth understanding formality levels in Spoken language. Because the spoken language is used much more often.
The first level of Spoken language is informal . Its use for routine messages delivered face -to-face with friends . Informal language is relaxed language with slang , contractions and humor .
The second level is Semiformal. Its use for important messages to strangers(for example, Phone call ). It is language with some contractions and personal pronouns.
And the last one is Formal . Its use for serious messages and in other formal contexts. It is correct, serious language using complete sentences without slang.
In a written language it is similarly important as in a spoken language, to what public this letter is directed.
Formality levels distribute on Intimate and Casual. Usually this is how writing to best friends or to friends and family members.
If you are writing a letter to a friend , writing something personal, you would use a more informal style.
And also distribute on Social, Formal and Professional.
If you are writing a letter for a job application or a academic essay , you would write in a more formal style.
Writing or Spoken language takes on different levels of formality depending on the social situation and the relationships between those involved.
It seems to me, these rules are very logical. And in my opinion, they are familiar to everyone.