GST mismatches: matching determines accepted input tax credit, duplicate/reversal adjustments, and filing timelines for liability. GST return matching classifies invoices, debit and credit notes into accepted input tax credit and mismatch/duplicate categories affecting output liability. Documents matched by filings due 20 October can be finally accepted and reclaimed with interest refunds; unresolved mismatches or duplicates increase liability in the current month's return. Forecast categories identify items that will increase liability in subsequent monthly returns if not rectified within the prescribed filing windows.