Re-categorize All (Transactions page header, tooltip Re-run auto-categorization on all unreviewed transactions) runs three steps:

1. Adds Sprwt's built-in starter keyword rules for common food-service suppliers and payment processors, skipping any keyword you already have.
2. Goes through every transaction that is not yet reviewed and applies the best matching rule.
3. Any unreviewed transaction with no matching rule is placed in Uncategorized Income or Uncategorized Expense (unless it was already auto-categorized in a previous run).

When finished you get a pop-up summary built from those numbers, one line per non-zero count, e.g. "3 default rule(s) added.\n12 transaction(s) matched a rule.\n5 transaction(s) assigned to Uncategorized — please review.\n2 skipped (already categorized)." Reviewed transactions are never touched. The button is only shown when Auto-categorize Transactions is set to Yes and you have the Edit Accounting Transactions permission.