As per Section 9 of the DEPOSITORY ACT 1996, Securities in depositories shall be in fungible form.
Here, Fungibility means " EASILY INTERCHANGEABILITY OF SECURITIES", held in depository form.
As we are aware that shares in physical form carry a distinctive number, to distingusih one share from another shares like we see on "currency notes".
But , when the physical shares r dematerialized, it results into lost of their distinctive identity number and that is called "FUNGIBILTY".
In fungibilty, investor has no right to obtain the exact certificate, he has surrendered at the time of entry into depository.
So fungibility, add more liquidity to securities and they become "INTERCHANGEABLE".
And that's the reason, In the depository system of maintaining securities ,S ecurities are required to be kept in fungible form.