The billing unit is rewritten words.
If an AI-writing report flags two methods paragraphs, you should not pay as if the whole paper were rewritten. HumanPen charges for words it actually changes after you import a Turnitin / iThenticate-style report (or select spans).
Citations, numbers, and formulas are treated as freeze targets. Free rounds on still-flagged spans after recheck are workflow hygiene—not a guarantee of institutional acceptance.