System reconciliation of GST returns mandates portal-matched adjustments to tax payable and input tax credit with ledger-based payments and interest. System-based reconciliation aligns data from FORM GSTR-1 and FORM GSTR-2 with FORM GSTR-3B: the portal auto-populates FORM GSTR-2A and drafts FORM GSTR-3, compares tax payable and ITC, requires additional payment by debiting electronic cash or credit ledger with interest where tax payable exceeds GSTR-3B, credits excess eligible ITC to the electronic credit ledger where applicable, and carries forward unreconciled reductions to subsequent returns; returns become valid only after payment of tax as per reconciled FORM GSTR-3.