System-based reconciliation of GST returns ensures adjustments to tax liability and ITC following auto-population and matching procedures. System-based reconciliation aligns FORM GSTR-1 and FORM GSTR-2 with FORM GSTR-3B by auto-drafting FORM GSTR-2A and Part A of FORM GSTR-3, allowing taxpayers to correct outward and inward supplies via GSTR-1 and GSTR-2. Where reconciled tax payable exceeds payment in GSTR-3B, additional tax must be paid by debiting the electronic cash or credit ledger with interest; where eligible ITC per GSTR-2 exceeds claimed ITC, excess is credited to the electronic credit ledger. Reductions in liability are carried forward if not offset; returns are valid only after full reconciled payment and statutory matching procedures apply.