What do the strength and readability settings do?

Strength controls how much changes. Light makes the smallest edits and keeps most of your phrasing; Balanced restructures sentences while keeping your terminology; Strong rewrites most freely. Balanced is the default and suits most work.

Readability sets the vocabulary level: Standard, University, Doctorate, Journalist, or Marketing. Use University or Doctorate for academic work to keep specialist terms; Journalist or Marketing for punchier prose. These map to the options the engine actually supports; there are no invented presets.