Tone of voice
Tone of voice is the situational adjustment a brand makes to its constant voice: warmer in a welcome, plainer in an apology, tighter in a status update. It shifts with audience, channel and circumstance while the underlying personality stays fixed, and it is usually documented as a small set of named situations.
Tone of voice is not Brand voice.
Voice is what does not change: the personality, vocabulary and habits that make writing recognisably yours. Tone is what does change: the setting you dial that voice to for a launch, a complaint or bad news. A brand has one voice and many tones, and treating the two as synonyms is why so many voice documents end up self-contradictory.
In practice
Almost every brand voice document fails in the same place. It tries to specify one register for every situation, discovers that the register is wrong for half of them, and gets quietly abandoned within a quarter. The fix is structural rather than editorial. Separate the constant from the variable, call the constant your voice and the variable your tone, and the document stops contradicting itself the moment you have to write something difficult.
Tone varies along a small number of dimensions, and naming those dimensions is far more useful than collecting adjectives. Formality: how close to spoken language you sit. Warmth: how much of the reader's feelings you acknowledge before getting to the point. Directness: how fast you reach the ask. Energy: how much lift the writing carries. A launch note might run high energy, moderate formality, very direct. An apology for an outage runs low energy, slightly more formal, still direct, considerably warmer. Both are the same voice at different settings.
The most common failure is a tone shift that is really a personality change. A brand that is dry and specific everywhere else turns exclamatory for a launch, and readers notice, because what changed was not the setting but the speaker. A rule of thumb worth keeping: if you have adjusted register and pace, that is tone. If you have adjusted vocabulary, sentence rhythm and your posture towards the reader, you have swapped voice, and something has gone wrong.
In practice, write four to six named situations rather than an abstract scale, because a scale is unusable at the moment of writing and a named situation is not. Welcoming somebody new. Announcing something you are proud of. Admitting you got it wrong. Explaining something technical to a non-technical reader. Give each a two-line description and one real sentence you would write alongside one you would not. That artefact survives handover to a new writer, and it is also the format an AI writing tool can read, which matters more every year: a generator handed a voice and no tone map will pick one default register and apply it cheerfully to a condolence.