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The top (yellow) rows control core norms governing social distance, dictating vous versus tu and Mr. Davis versus Davis. Talkers (column 1) converse, directly and personally; whereas writers (columns 2 - 4) address their broad audiences impersonally (indirect personal reference).
As the green category reflects, writing more than speech conforms to syntactical rules; exaggerating and inventing them increases formality (hyper-grammatical rules).
Conversational speech is naturally prolix; formality emphasizes succinct expression (succinctness over naturalness).
Finally, talkers' propinquity encourages unique expression. Banishing terms known only to a profession or other special group, Formal-level writing aspires to literate universality, and the Ceremonial level appeals still more broadly to a humanity-wide quasi-musical sensibility (expressive universality).
The Informal level is more conversational than the Formal because difference from speech creates distance formality, but that isn't to prejudge any prescriptive questions. The next entry will answer questions like should legal-brief writers (or all writers) be as informal as the applicable social-distance norms permit?
Next entry: Part 3. Choice of register