HumanFlow offers 3 tones: natural, casual and professional. There is no custom voice option.
Natural — Sounds like you, unforced.
Casual — Warmer, closer to speech.
Professional — For work and academic writing.
The trade is deliberate. Three tones that behave predictably were judged more useful than a longer list where telling two apart requires running both. Some competitors offer nine or more modes, and if fine control over register is what you want, that is a real advantage they have and we do not.
Tone affects word choice and sentence construction. It does not change the argument, reorder the structure, or add anything that was not in your draft.
Pick the tone that matches where the writing is going rather than where it came from. A stiff draft heading for a personal blog wants Casual even though the source reads formally.
If you are unsure, run a representative section through two tones and compare. That costs a few hundred words of allowance and answers the question better than any description here.
Tone is applied to the whole submission rather than per paragraph. If a document needs different registers in different sections — a formal introduction and a conversational conclusion — process those sections separately with different tones and reassemble them.
The tones are named in one place in the code and read from there by every page that mentions them, which is why you will not find a list here that disagrees with the picker. Earlier versions of this site advertised tone names the product had never offered, and deriving them rather than typing them is how that stopped happening.