A Legal Guardian Letter is an official document issued by a court designating an individual as the legal guardian of a minor. This letter serves as proof of the individual's authority to act on behalf of the minor in financial and legal matters, including opening and managing a minor's bank or investment account. 

As per SEBI regulations, a guardian must manage a minor’s account until they attain a majority. If the guardian is not the biological parent (father or mother), a Legal Guardian Letter is mandatory to establish the legal relationship and ensure regulatory compliance for operating the minor’s account.