Depending on your study history, you may have learned hundreds (or even thousands) of words before starting with Hack Chinese. In your onboarding questionnaire or manually, these words become assumed known.
Words that are assumed known are treated differently from unknown words in two ways:
If you'd like to check your assumptions (and eliminate gaps in your knowledge) quickly, you can set the assumption check rate to 'Fast'. You'll check at most 30 assumed known words per day.
If you'd like to focus on learning new words (with the risk of letting potential gaps in your knowledge persist longer), you can set your assumption check rate to 'Slow'.
You can also 'pause' the checking (and never review words that are assumed known) or check all of them (with unlimited mode).