Section 13(3) of the Income-tax Act, 1961, specifies the persons who are considered to have a substantial interest in a trust or institution for the purposes of determining whether the income of the trust or institution is eligible for tax exemption. This section is crucial in identifying related parties whose transactions with the trust or institution could potentially lead to the denial of tax exemptions under Section 11 or 12. The statutory test involves examining the relationship and financial transactions between the trust and these specified persons. The burden of proof lies on the trust to demonstrate that no undue benefit has been conferred upon these persons. This section is significant as it ensures that tax-exempt entities do not misuse their status to benefit specific individuals or entities, thereby maintaining the integrity of charitable organizations.