Answer first
What information should be redacted in discovery?
Typically personal identifiers the rules protect, Social Security numbers, taxpayer IDs, birth dates, names of minors and financial account numbers, plus privileged content and anything a protective order covers. Check your court's rules for the exact list.
In US federal courts, FRCP 5.2 limits filings to the last four digits of SSNs, taxpayer IDs and financial account numbers, the year of birth, and a minor's initials. Many state courts have similar rules.
Productions often also need third-party personal data, medical information and confidential business details removed, depending on the protective order and privacy laws that apply.
Re-Doc finds these identifiers across the whole production and either redacts them permanently (text is removed from the file, not covered) or replaces them with consistent synthetic values when the document must stay readable.