A voice is more useful as rules and examples than as adjectives. “Friendly” means something different to every writer. “We say ‘you’, never ‘customers’, and we never use exclamation marks” means the same to all of them.
Voice keeps a brand recognisable when several people write for it. Tone changes with the situation: lighter for a new menu, plainer for a delayed order, but in the same voice.
Example
A bakery's voice rule: short sentences, the names of the bakers, no “artisanal” and no “delicious”. Show the bread, don't praise it.