Dematerialisation is the process by which physical share certificates of an investor are converted to an equivalent number of securities in electronic form and credited into the investor’s account maintained with his/her depository participant (DP). It is like having a bank account where instead of money, you hold securities in your account.
What is an ISIN?
ISIN (International Securities Identification Number) is a unique identification number for a security. India follows the norms stipulates by Association of National Numbering Agency (ANNA) which is the international body for issue of ISINs. National Securities Depository Limited (NSDL) issues ISINs in India and a complete list of ISINs is available on their website: www.nsdl.co.in. Also note that ISIN of a security changes in case of certain corporate action such as split in share par value, consolidation of share capital etc. Hence you have to quote the correct ISIN at the time of giving dematerialisation request as well as at the time of transfer of shares.

Do dematerialised shares have distinctive numbers?
Unlike physical shares, dematerialised shares do not have any distinctive numbers. These shares are fungible, which means that all the holdings of a particular security will be identical and interchangeable.