Return of income filing rules set mandatory classes, due dates, updated return limits, and defect rectification safeguards. Filing of return of income is mandatory for specified classes of assessees, including companies, firms, persons crossing the basic exemption limit, specified entities, persons carrying forward losses, and residents with foreign assets or signing authority in foreign accounts, with different due dates prescribed for audited and non-audited cases. The Board may prescribe the form, verification, electronic filing requirements, and additional particulars such as exempt income, foreign assets, bank accounts, expenditure, audit reports, and business or partner details, while the Central Government may exempt classes of persons from filing. The section also provides for belated, revised, and updated returns, subject to detailed time limits and restrictions, and treats non-conforming returns as defective unless rectified within the allowed period.