Tone and register

Matching the situation.

Formality

Formality is set by the relationship and the stakes, not by long words. Contractions and direct address are fine in most business writing and out of place in a legal filing.

Read it back as if you were the recipient having a bad day.
Humour and irony travel badly in writing and across cultures.

Inclusive and plain

Prefer terms people use about themselves, avoid unnecessary gendering, and expand an abbreviation on first use. It costs nothing and removes barriers.