SEBI has vide its circular dated 4th January, 2019 clarified that in case of transmission of shares held in demat mode, it shall follow the same procedure in respect of documentation as is specified in case of transmission of shares in physical mode. In physical mode, it has been specified that succession certificate or probate of will or will or letter of administration or court decree etc. as may be applicable in terms of the Indian Succession Act, 1925 are the documentary requirements.
Now in terms of the aforesaid circular, the same documentary requirements shall be applicable to transmission of securities held in demat made. So basically the successors or heirs have to produce the above set of documents to prove their title and submit the same to the depository participant.
Copy of SEBI circular is enclosed here