Wiping permanently deletes all your data for every book — including genres you've set, "know the author" flags, and recommendations. Leave this unchecked to safely re-sync an updated Goodreads export: existing books are matched and updated in place. Every field — genre, title/author, shelf, rating, dates, notes and reviews — is only ever filled in when it's currently blank (an untouched book still sitting on "to-read" counts as blank, so finishing it on Goodreads and re-syncing still updates it here). Anything you've already set in Releasing Lions always wins over the CSV — and only genuinely new books get added.