GSTR-1 return sets the monthly reporting format for outward supplies, invoices, credit/debit notes, exports, and e commerce sales. Form GSTR-1 prescribes a monthly structured return for reporting outward supplies, organised into tables for supplies to registered persons, high value inter state consumer supplies, supplies to consumers, credit/debit notes, nil/exempt/non GST supplies, exports, time of supply liabilities without invoice, tax paid on advances, and supplies via e commerce portals, with amendment sections, HSN/SAC reporting thresholds, and a required authorised declaration.