ISIN stands for International Securities Identification Number — a 12-character alphanumeric code (e.g., INE002A01018 for Reliance Industries) that uniquely identifies a security globally. The ISIN is allocated by the depository (NSDL or CDSL in India) typically a few days before the listing date.
Indian ISINs begin with 'INE' followed by a 9-character identifier. The ISIN is the field your demat account uses to reference the security — when you check your portfolio, the underlying record is keyed on ISIN, not on the more familiar trading symbol or company name.
For IPO research, the ISIN is rarely needed before listing. It becomes relevant after listing when checking allotment status, transferring shares between demat accounts, or tracking corporate actions like bonus issues and stock splits. The RHP typically mentions that the ISIN is 'to be allotted' before the issue closes.