Tone of voice is the consistent personality of a brand's language — how it sounds, not what it says.
Useful tone-of-voice guidance in a brief is comparative and concrete: "confident but not boastful", "plain German, no anglicisms", "we say you, never one". Adjectives alone ("modern, friendly, dynamic") give a writer nothing to act on.
The most practical format is a short do/don't pair with a rewritten example sentence.