The Basis of Allotment (BoA) is the formal document published by the registrar to the issue, with approval from the designated stock exchange, detailing how the available shares have been distributed across applicants. It is the legal record of the allotment outcome.
For undersubscribed or fully-subscribed-but-not-oversubscribed issues, the BoA is straightforward — every eligible applicant gets allotted what they bid for, subject to category caps. For oversubscribed issues, the BoA explains the methodology applied: lottery for retail in mainboard IPOs, proportional for QIB and NII, and the specific ratio applied (e.g., '1 in 4 retail applicants got allotted').
The BoA is published on the registrar's website and on the exchange's IPO page on the basis-of-allotment day, typically 3 working days after the issue closes. Retail investors can read the BoA to understand exactly why they did or did not get allotted, and how oversubscribed each category was at the final settlement price.